CBL and CIPK homologs were identified using BLASTp and tBLASTn searches of the Uniprot and the NCBI protein and nucleotide databases, using previously identified CBLs and CIPKs from Arabidopsis as queries. Additional sequences were manually retrieved by annotation from UniProt using the keywords “calcineurin” and “CBL-interacting” (Jain et al., 2009). Genomic loci of CBL and CIPK homologs in Physcomitrella patens were identified in version 1.6 of the Physcomitrella genome, available at http://cosmoss.org (Zimmer et al., 2013). All charophyte CBL and CIPK sequences identified were predicted by assembly of homologous expressed sequence tags (ESTs) from transcriptome-level sequencing of diverse, representative charophyte genera (Timme and Delwiche, 2010; Timme et al., 2012). Other new CBL and CIPK protein sequences were predicted from EST sequences in the NCBI non-redundant (nr) nucleotide database identified by tBLASTn searches. Overlapping ESTs from the same taxa were assembled, and ESTs were translated using Geneious R6 (Biomatters), which was also used for all stages of phylogenetic analyses and figure preparation. Predicted CBL and CIPK homologs were verified by manual inspection of domain architecture and pBLAST searches of the NCBI non-redundant (NR) protein database; all protein sequences included in analyses showed expected domain architecture and yielded top BLASTp hits to previously identified CBLs and CIPKs. CBL and CIPK homologs identified in this study are listed in Supplementary Tables S1,S2, respectively. Protein sequences were aligned using MAFFT (algorithm G-INS-i) and edited and trimmed by eye to remove short, ambiguously aligned regions (see Supplementary Files S1,S2). Edited alignments were used to generate the phylogenetic trees shown Katoh et al. (2002). Phylogenetic trees were generated from the resulting multiple sequence alignments (MSAs) using PhyML with subtree pruning and regrafting (SPR) + nearest neighbor interchange (NNI) moves and X²-like approximate likelihood ratio test (aLRT) clade support values, which serve as confidence scores much like bootstrap scores. Clades with aLRT scores > 0.95 were deemed to have strong phylogenetic support (Anisimova and Gascuel, 2006; Guindon et al., 2009). Specific model parameters are provided in the figure legend for each PhyML analysis presented, however several additional MSAs and evolutionary models and parameters were tested for agreement with conclusions presented here (data not shown). Clades and evolutionary relationships mentioned in the text appeared consistently in independent phylogenetic analyses with different model parameters and MSAs.

In order to verify expression and expected splice patterns of CBLs and CIPKs in an early-diverging land plant, we cloned CBLs and CIPKs identified from the model moss Physcomitrella. RNA was extracted from protonema and gametophores of Physcomitrella patens ssp patens ecotype Gransden 2004 using a CTAB/chloroform method similar to the one described by Chang et al. (1993). The RNA was reverse transcribed to produce cDNA using Superscript III Reverse Transcriptase (Invitrogen). Primers containing Invitrogen Gateway attB1 (forward primers) and attB2 (reverse primers) recombination sites were designed to amplify the coding sequences (CDSs) of each Physcomitrella CBL (PpCBL) and CIPK (PpCIPK) genes (see Supplementary Table S3 for oligonucleotide sequences used in this study). CBL and CIPK transcripts were amplified using Phusion DNA Polymerase (Thermo-Fisher Scientific) following recommended manufacturer protocols on a MJ Research PTC-100 or PTC-200 model thermocycler. PCR products were visualized on a 0.8% agarose gel, and products of the expected sizes were extracted using a QIAquick gel extraction kit (Qiagen) and cloned into the pDONR™/Zeo vector (Invitrogen) by Gateway BP reaction, following manufacturer recommendations. Samples from three or more clones for each gene were submitted to Elim Biopharmaceuticals, Inc. (Hayward, CA) for DNA sequencing.

In order to verify physical interactions among CBLs and CIPKs in a non-angiosperm plant, we cloned the CDS of each full-length CBL and CIPK transcript identified in Physcomitrella and tested interactions among PpCBLs and PpCIPKs in yeast two-hybrid (Y2H) assays using the yeast strain AH109 (Clontech Inc.). This strain is auxotrophic for leucine, tryptophan, histidine, and adenine. The CDSs of PpCBLs and PpCIPKs were cloned by Gateway LR reaction into yeast two-hybrid gateway-compatible vectors (pGBT9-BS-GW and pGAD-GH-GW) derived from pGBT9-BS and pGAD-GH (Clontech). These vectors were transformed into yeast cells using the G-Biosciences FastYeast Transformation Kit and used to express CBL and CIPK fusions to the DNA-binding domain (BD) and activation domain (AD) of a split transcription factor. We screened CBL-BD fusions (pGBT9-BS-GW constructs) for interactions with CIPK-AD fusion proteins (pGAD-GH-GW constructs) and performed reciprocal screens among CIPK-BD and CBL-AD fusion proteins to verify that the interactions were not vector-dependent. As negative controls, we verified that CBL-BD or CIPK-BD fusion proteins did not interact with the pGAD-GH empty vector (EV).

To perform Y2H screens, co-transformed cells were cultured to mid-log phase in MP Biomedical drop out base (DOB) liquid media lacking leucine and tryptophan (-LT), to ensure retention of vectors containing bait and prey constructs. We then adjusted the cultures to OD₆₀₀ = 0.05 and divided them into four 10-fold dilutions (OD₆₀₀ = 5 × 10⁻², 5 × 10⁻³, 5 × 10⁻⁴, 5 × 10⁻⁵). 6 μl droplets of each dilution were plated on agar DOB media (1) lacking leucine and tryptophan (-LT) to serve as a positive control for transformation and loading, (2) lacking leucine, tryptophan, and histidine (-LTH) to test for protein-protein interactions under low stringency, and (3) lacking leucine, tryptophan, histidine, and adenine (-LTHA) to test for interactions under stringent conditions. Cell growth was recorded at 48 h intervals over the course of 6 days.

CBLs and CIPKs have been previously identified among various land plants and chlorophyte green algae, though other chlorophytes appear to lack CBL-CIPK homologs (Weinl and Kudla, 2009). Utilizing recently available transcriptome data, we identified CBL and CIPK homologs from several charophyte green algae species: Coleochaete orbicularis, Klebsormidium flaccidum, Chaetospheridium globosum, Penium margaritaceum, and Chlorokybus atmophyticus. Interestingly, we identified a single CBL and single CIPK in each of these lineages, with one exception. We could not confidently identify a CIPK homolog from Chlorokybus, though this may due to incomplete transcriptome coverage. Additional CBL or CIPK homologs may be present in these taxa but undetected due to incomplete sequencing coverage, or additional homologs may simply not be transcribed at sufficient levels under sampled growth conditions. In agreement with our current understanding of evolutionary relationships among these organisms, charophyte green algae sequences display greater sequence similarity to land plant CBLs and CIPKs than chlorophyte homologs. Although there is no currently available genome sequence for any charophyte, only a single CBL and single CIPK were identified in the complete genome sequence of the chlorophytes Ostreococcus lucimarinus and Bathycoccus prasinos, consistent with prior findings (Weinl and Kudla, 2009). Though it is difficult to make genomic inferences about any charophyte green alga without an available complete genome sequence, our analyses suggest that green algae commonly contain a single CBL-CIPK pair and that the CBL-CIPK network likely predates the split of chlorophyte and charophyte algae.

All CBLs and CIPKs analyzed in this study, including the most divergent homologs identified in algae, show strong conservation of domain architecture and important motifs. At approximately 200 amino acids (AAs) in length, CBLs contain one of a few variations of a localization at their N-termini, followed by 4 calcium-binding EF hand domains. The first EF-hand of CBLs is distinctive in that the calcium-binding loop is comprised of 14 rather than 12 AAs, however evidence suggests that it indeed binds calcium ions (Nagae et al., 2003). Identified full-length CIPKs are approximately 475 AAs in length and have a conserved domain architecture comprised of a N-terminal kinase domain and a C-terminal autoinhibitory region with a diagnostic NAF domain that mediates interactions with CBLs. One previously identified CIPK from the chlorophyte green alga Chlorella (UniProt: C4P7Q5) differs, however, in that it possesses 2 NAF domains in its C-terminus, though the significance and accuracy of the published domain architecture is unknown. Our homology search results corroborate the assertion that CBLs and CIPKs are not found in certain chlorophyte green algae, including the models Chlamydomonas and Volvox (Weinl and Kudla, 2009; Batistic et al., 2011). This pattern parallels trends in calcium channel evolution. The Chlamydomonas genome encodes several voltage-dependent calcium channels (VDCCs) and transient receptor potential (TRP) channels, which play critical roles in environmental sensing in metazoans, whereas sequenced land plant genomes do not contain discernible homologs from either family (Wheeler and Brownlee, 2008; Verret et al., 2010). Like most metazoans, Chlamydomonas is motile and, in addition to performing photosynthesis, readily grows heterotrophically. Chlamydomonas cells contain an organelle unlike any found in plants, the eyespot, that is involved in the calcium-mediated process of phototaxis (Witman, 1993). Based on these observations, it appears that some components of the calcium signaling machinery of certain chlorophyte green algae, such as Chlamydomonas, more closely resemble animal signaling networks in some aspects than those of land plants.

Taking advantage of the published genome sequence of the moss Physcomitrella patens, we determined the genomic complement of CBLs and CIPKs in this early-diverging model plant. We identified a total 4 CBL and 7 CIPK predicted protein sequences in Physcomitrella, consistent with prior reports (Batistič and Kudla, 2009; Weinl and Kudla, 2009). One pair of CBLs (PpCBL2+3) and three pairs of CIPKs (PpCIPK1+5, 3+4, and 6+7) showed strikingly high sequence similarities at both the amino acid (73–93% pairwise identity) and genomic level (42–52% pairwise identity). Because of this observation and the inferred whole genome duplication (WGD) estimated to have occurred ~45 million years ago in Physcomitrella (Rensing et al., 2007), we hypothesized that pairs of CBLs and CIPKs are products of the recent WGD and that the “unpaired” CBLs (PpCBL1 and PpCBL4) and CIPK (PpCIPK2) may similarly possess cognate loci in the Physcomitrella genome. Consistent with this hypothesis, we identified paired loci for each gene and provisionally named these PpCBL5, PpCBL6, and PpCIPK8 (Figure 1). Although these loci showed relatively low percentage identity to their cognate loci compared to previously detected CBLs and CIPKs, gene predictions using Augustus (Stanke et al., 2004) suggested these loci may encode partial or full-length proteins. Using RT-PCR, we amplified and cloned transcripts from PpCBL5 and PpCIPK8, however we failed to amplify transcripts from the PpCBL6 locus using several primer pairs validated on genomic DNA (data not shown), despite testing cDNA from different developmental stages (protonema, gametophores, sporophytes). Pairwise alignment of the PpCBL4 and PpCBL6 loci revealed a relatively low percentage identity, particularly in PpCBL4 exonic regions, compared to other “sister” pairs of CBLs and CIPKs; these observations suggest that PpCBL6 may be a pseudogene. Sequenced PpCBL5 and PpCIPK8 transcripts detected from both gametophyte and sporophyte cDNA were found to contain premature termination cassettes (PTCs) in their spliced forms (see Figure 1, Supplementary File S3), which suggests that these transcripts may not be translated, at least under conditions that we tested. Physcomitrella CIPK8 contains a single nucleotide repeat (SNR), which are known to promote mutations and quickly change in length (Ellegren, 2004), spanning 31 bases in the retained PTC in cloned transcripts, further marking it as an unusual CIPK. Interestingly, PpCBL5 and PpCIPK8 show obviously stronger conservation in exonic regions (i.e., regions retained in spliced transcripts and that align to their sister gene's CDS) than intronic or regulatory regions (i.e., promoter and terminator). While these aberrant CBL and CIPK loci may simply be in early stages of pseudogenization, the unexpected finding that these loci are transcribed and spliced warrants further investigation into possible functions and may point toward a role for these transcripts as regulatory RNAs, as shown previously in animals (Korneev et al., 1999; Hirotsune et al., 2003; Tam et al., 2008). Like in Physcomitrella, expansion of the CBL-CIPK network in Arabidopsis, previously attributed to segmental duplications (Kolukisaoglu et al., 2004), can be traced to known WGD events in light of our current understanding of plant genome evolution (Cui et al., 2006). Independent expansion of other gene families in moss and angiosperms has been described, and this can obfuscate direct comparison and functional prediction of genes in widely divergent plants (Cui et al., 2006; Bowman et al., 2007; Rensing et al., 2008; Jiao et al., 2011).

Phylogenomic methods extend the ability to determine relationships among distant homologs, facilitate functional prediction, and provide a framework for discovery of key features by identifying conserved regions of proteins (Eisen and Wu, 2002; Sjölander, 2004). Using maximum likelihood (ML) methods, we reconstructed the phylogeny of the CBL family in green lineages. Consistent with our hypothesis that land plant CBLs and CIPKs expanded from a simple module present in their common ancestor with algae, green algal CBLs cluster closely to one another with high confidence scores in phylogenetic analyses. Although algal CBLs do not consistently cluster with any particular clade of CBLs from land plants, they commonly show moderate phylogenetic affinity for a clade containing Arabidopsis CBL1 and CBL9 (Figure 2; see Supplementary Figure 1 for full tree), which play important roles in potassium nutrition through regulation of the AKT1 channel. Like Arabidopsis CBL1 and CBL9, green algal CBLs feature the dual-lipid modification motif MGCXXS/T or obvious relicts of this motif. Due to the retention of this motif among many embryophyte and green algal CBLs and the results of our phylogenetic analysis, we hypothesize that the dual-lipid modification motif is the ancestral localization mechanism of CBLs. This hypothesis is strengthened by the observation that distantly homologous neuronal calcium sensor (NCS) proteins feature a similar N-terminal motif (MGXXXS) that lacks the conserved cysteine residue but does trigger N-myristoylation of the conserved glycine residue (Li et al., 2011). We designate homologs with the dual lipid modification motif as Type I CBLs (Figure 3 top). Consistent with the hypothesis that ancestral CBLs most closely resembled modern Type I homologs and gave rise to other types of CBLs, Type I CBLs are paraphyletic with respect to other CBLs. Arabidopsis CBLs containing the Type I dual lipid modification motif have been shown to localize to the plasma membrane (D'Angelo et al., 2006; Cheong et al., 2007; Batistic et al., 2008). Mutational analyses using FP-fusions indicate that both N-myristoylation and S-acylation are required to target proteins to the plasma membrane, whereas either modification on its own results in endomembrane localization (Batistic et al., 2008). Although subcellular localization has not been investigated in early diverging plants or green algae, we speculate that the ancestral CBL-CIPK module may have participated in the regulation of integral membrane proteins at the plasma membrane, given the observed evolutionary trends and our understanding of CBL-CIPK function and biochemistry in Arabidopsis.

Phylogenetic analyses revealed a strongly supported (aLRT = 1.0) clade that contains Physcomitrella CBL2 and CBL3 and Arabidopsis CBL2, CBL3, CBL6, and CBL7 (Figure 4). Physcomitrella CBL2 and CBL3 encode proteins that are 76% identical, and both genes lack introns, unlike other CBLs from Arabidopsis or Physcomitrella. The clade also contains homologs from other non-angiosperms, including three CBLs from the lycophyte Selaginella moellendorffii. Like Arabidopsis, Selaginella CBLs in this clade contain multiple introns and exhibit a conserved exon-intron structure (data not shown), leading us to infer there was a likely reverse transcription event in the Physcomitrella lineage not shared with the lineage leading to lycophytes and angiosperms. Experimental work is needed to determine functional consequences of intron loss in Physcomitrella CBL2 and CBL3, however the effects and mechanisms of reverse transcription-mediated intron loss events and other means of intron loss are discussed elsewhere (Jeffares et al., 2006; Filichkin et al., 2010). Based on high sequence similarity, shared intron loss, and strong phylogenetic evidence, we infer that PpCBL2 and PpCBL3 are products of a lineage-specific gene duplication, likely the results of a recent WGD (Rensing et al., 2007, 2013). Both genes are orthologous to the four Arabidopsis CBLs contained in this clade. Arabidopsis CBL2 and CBL3 are also recent duplicates, as evidenced by their phylogenetic placement and very high sequence similarity (~92% AA identity) throughout their entire lengths. CBL3 and CBL7 are tandem duplicates, although CBL7 is disparate from other Arabidopsis CBLs and contains a deletion in its N-terminus between a degenerate dual-lipid modification motif and its first EF hand (Batistič and Kudla, 2009). Arabidopsis CBL6, which features an unusual first EF hand relative to other CBLs, is more distantly related to the three other AtCBLs in this clade and forms a clade with orthologous CBLs from other eudicots.

Arabidopsis CBL2, CBL3, and CBL6 have been reported to localize to the tonoplast (Batistič and Kudla, 2009). In the case of CBL2 and CBL3, it has been rigorously shown that an N-terminal motif known as the tonoplast targeting sequence (TTS) mediates its subcellular localization (Tang et al., 2012). The TTSs of Arabidopsis CBL2 and CBL3, with the consensus motif MSQCXDGXKHXCXSXXXCF, span 19 AA; and the last three positions of the motif overlap with positions 2–4 in the dual lipid modification motif of Type I CBLs (i.e., MGCXXS/T), sharing a conserved cysteine residue found in all CBLs analyzed (see Figure 3, Supplementary File S1). This 19-AA fragment from either Arabidopsis CBL2 or CBL3 is necessary and sufficient for targeting of FP fusions to the tonoplast in Arabidopsis mesophyll cells (Tang et al., 2012), and strong sequence similarity suggests that CBL6 shares this targeting mechanism (Figure 3 middle). CBL7 is reported to show a diffuse nuclear and cytosolic localization based on the analysis of fluorescent fusion proteins (Batistic et al., 2008), however we are unaware of any rigorous attempts to determine its subcellular localization. Therefore, it appears that tonoplast localization is a generally conserved feature among angiosperm CBLs in this clade. We identified a TTS-like motif in all three Selaginella CBLs in this clade and in PpCBL3. Unlike PpCBL3, PpCBL2 does not contain an extended N-terminus and instead contains the Type I dual lipid modification motif. Given these trends, we posit that the TTS is a synapomorphy of this clade and that PpCBL2 lost its TTS via deletion or partial gene conversion, as described elsewhere (Jeffares et al., 2006). Based on strong phylogenetic support and TTS motif conservation, we designate homologs contained in this clade as Type II CBLs.

Phylogenetic analyses also identified a strongly supported clade that contains Arabidopsis CBL10, the only Arabidopsis CBL predicted to contain a transmembrane (TM) helix for membrane association (Figure 5). This clade contains orthologs from all studied angiosperms and gymnosperms, indicating this clade is conserved among seed plants; and all members of this clade with full-length sequences exhibit a predicted N-terminal transmembrane helix. Like members of the AtCBL10 clade, Physcomitrella CBL4 contains an extended N-terminus, which we posited may form a transmembrane helix (Figure 3 bottom). Various TM topology prediction methods disagree on whether AtCBL10 or PpCBL4 contain a predicted TM helix (data not shown), however visual inspection of hydrophobicity and patterns of conservation in MSAs suggests that both AtCBL10 and PpCBL4 contain N-terminal TM helices. The presence of this TM helix raises the possibility that PpCBL4 may be an AtCBL10 ortholog, however our phylogenetic data neither favor nor disfavor this hypothesis. More thorough coverage of sequence data from early-diverging plants is likely required to test this possibility and determine whether Type III CBLs are monophyletic or not. The Arabidopsis CBL10 transcript is reportedly processed into mRNAs that encode proteins with two distinct N-termini, though both share the same TM helical region. Alternative splicing is mediated by a unique 8th intron (other rice and Arabidopsis CBLs contain 6 or 7 introns) toward the 5′ pend of the transcript. Both Physcomitrella CBL4 and Arabidopsis CBL10 share a very similar exon-intron structure (data not shown), though we did not find evidence of alternative splicing in PpCBL4.

The typically short length and strong structural conservation of EF hand proteins like CBLs can complicate phylogenetic reconstruction, as relatively few substitutions can significantly influence results. Due to biophysical constraints, EF hand domains typically exhibit strong sequence conservation at positions that coordinate calcium ion binding. However, variation seen among EF hands of CBLs are predicted to have widely differing affinities for calcium ions, thereby facilitating functional diversity at the level of calcium binding. The 4th EF hand (EF4) of Physcomitrella CBL4 is unusual in that it contains non-polar residues at two of the positions that coordinate calcium ion binding, rather than negatively charged residues as seen in virtually all other EF hands. Therefore, it appears likely that it does not bind calcium. Indeed, studies of other calcium signaling pathways have underscored the plasticity of signaling components during evolution. The model yeast Saccharomyces cerevisiae contains a single-copy gene encoding a calmodulin (CaM), a widely studied type of calcium sensor in eukaryotes. This gene, CMD1, is indispensable for survival of the cell. Surprisingly, molecular genetic analysis suggests the CaM's ability to bind calcium ions is dispensable for its most vital functions, and its fourth EF hand is unable to bind calcium (Cyert, 2001). Plants contain a suite of typical CaMs and widely divergent CaM homologs, some of which either lack the ability to bind calcium ions or coordinate them in an unusual manner (McCormack and Braam, 2003). Further work is needed to clarify the capacity and affinity of identified CBLs for calcium binding, particularly among non-angiosperm CBLs.

There has been some debate as to the localization of Arabidopsis CBL10; various reports indicate localization to the tonoplast (Kim et al., 2007; Batistic et al., 2010) or plasma membrane (Quan et al., 2007; Ren et al., 2013). Although the multiple isoforms of CBL10 may account for different localization patterns, Arabidopsis CBL10 is most strongly expressed in shoots and is suggested to participate in the regulation of a NHX-family, Na⁺/H⁺ exchanger believed to function in the sequestration of sodium ions within the vacuole. A model has emerged wherein CBL10 plays a regulatory role in the SOS pathway akin to that of CBL4/SOS3 (Kim et al., 2007; Tang et al., 2013). In root hair and cortical cells, the Type I Arabidopsis CBL4 forms a complex with CIPK24, and together they regulate the activity of the plasmalemma-localized Na⁺/H⁺ exchanger SOS1 (syn. NHX7) and facilitate the extrusion of sodium ions from the plant. In shoot mesophyll cells, CBL10 complexes with CIPK24, and together they putatively regulate the activity of an unidentified tonoplast-localized Na⁺/H⁺ exchanger and facilitate sequestration of sodium ions in the vacuole. A recent publication proposes a role for CBL10 in the regulation of the plasmalemma-localized potassium channel AKT1 (Ren et al., 2013), which has been rigorously shown to be subject to regulation by CBL1 and CBL9 acting in concert with CIPK23 (Li et al., 2006; Xu et al., 2006). Our phylogenetic results indicate that the single-pass N-terminal TM helix is a synapomorphy of the AtCBL10 clade. Physcomitrella CBL4 likewise contains a N-terminal TM helix and may be orthologous, therefore we designated these homologs Type III CBLs.

Different membranes of the eukaryotic cell have distinct phospholipid profiles, which can serve as a basis for subcellular targeting. Moreover, each particular membrane is commonly composed of distinct microenvironments with unique lipid and protein populations. Together, proteins and lipids are thought to form functional modules in cellular membranes, with membrane-targeted kinases recognized as common regulatory modules (Engelman, 2005). For these reasons, we expect that CBL-CIPK complexes are likely targeted not only to specific membranes but to precise sites within membranes where they interact and function with molecular partners (Bhatnagar and Gordon, 1997; Levental et al., 2010). Elevation of free calcium in the cytosol is localized and transient, partly due to effects of Ca⁺⁺-ATPases and Ca⁺⁺/H⁺ antiporters and proteins that act as buffers. Because calcium signatures occur locally, calcium sensors must operate in close proximity to the channels responsible for calcium elevation (Fogelson and Zucker, 1985; Gilroy et al., 1993; Roberts, 1994; Clapham, 2007). In light of this, we interpret the conservation of CBL localization motifs among distantly related plants as a likely consequence of constraints on CBL-CIPK subcellular localization.

Although several studies have examined CBL localization, it remains unclear whether CBLs display a predominantly static or dynamic localization at protein maturity. Our analyses demonstrate that the cysteine residue occupying the third position in the Type I motif (MGCXXS/T) is perfectly conserved among CBLs from widely divergent organisms and paralogous clades. In Type I and Type II CBLs, this residue has been shown to be S-acylated, and the modification is required for known protein functions (Batistic et al., 2008; Batistič, 2012; Tang et al., 2012). Based on its striking conservation, we predict that S-acylation of this conserved residue is a shared among CBLs, at least under certain conditions. It is well established that S-acylation is a reversible post-translational modification and that it can strongly impact protein localization and can be critical for protein function (Bijlmakers and Marsh, 2003; Hemsley and Grierson, 2008). Prior research has pointed toward a role for S-acylation in fine-level targeting of proteins to specific membrane microenvironments (Bhatnagar and Gordon, 1997; Mumby, 1997; Dunphy and Linder, 1998; Levental et al., 2010). We predict that N-terminal S-acylation at this conserved residue functions, at least in part, as a mechanism for precise and dynamic localization of CBLs.

The CBL-CIPK network is mediated by a conserved CBL-interacting domain (also known as the NAF or FISL motif) in CIPKs. Our MSA of the CIPK family indicates that the NAF domain is strongly conserved, with many identical residues, among algal CIPKs and all CIPKs from Arabidopsis and Physcomitrella (Figure 6). This observation is consistent with our prediction that CBLs and CIPKs from green algae and early diverging embryophytes function together as a module. To confirm our presumption that Physcomitrella CBLs and CIPKs physically interact with each other and lend support to our interpretation of these protein families as functionally connected in early-diverging plants, we performed Y2H screening and characterized physical interactions between full-length PpCBLs and PpCIPKs in yeast cells.

Consistent with our expectations, CBLs and CIPKs from Physcomitrella showed physical interactions in yeast cells. All combinations of PpCBL and PpCIPK fusion proteins showed physical interactions in yeast (Supplementary Figure S2), but specific CBL-CIPK combinations showed very strong interactions with select partners, consistent with the hypothesis that particular CBLs show preferential interactions with cognizant CIPKs (Figure 7). We observed that “creeter” CIPKs displayed overlapping, though not identical, interaction profiles with their most closely related homolog. CIPK1 and CIPK5 interact moderately with CBL2 and CBL4 and weakly with CBL1 and CBL3. CIPK3 and CIPK4 interact weakly with CBL3 but moderately to strongly with CBL1, 2, and 4. CIPK6 and CIPK7 interact strongly with CBL4 and weakly to moderately with CBL1, 2, and 3. We observed only weak interactions between CIPK2, which lacks a “sister” CIPK, and any CBL, despite conservation of its NAF domain and phylogenetic proximity to the highly interactive CIPK3 and CIPK4.

Among the CBLs, CBL4 shows the highest number of strong connections to CIPKs, and it interacts very strongly with CIPK6 and CIPK7, members of the green algal clade of CIPKs. CBL1, a Type I CBL without clear phylogenetic affinities to angiosperm CBLs, most strongly interacts with CIPK4 and shows very weak interactions with CIPK1 and CIPK5. CBL3 shows clearly weaker interactions with CIPKs than its close paralog CBL2, although their interaction profiles are similar. CBL2 and CBL3 both interact most strongly with CIPK3, CIPK4, and CIPK6. These data support the model of a highly interconnected signaling network, however interaction patterns may differ significantly in moss cells due to differences in post-translational modifications, subcellular localization, expression, and other factors. Nonetheless, these results provide a guide for genetic analyses in moss and lend confidence to the interpretation that CBLs and CIPKs are functionally linked in early-diverging plants and constitute an ancient signaling network.

Phylogenomic analyses of the CIPK family were pursued, as described for CBLs, to decipher evolutionary patterns to facilitate identification of functionally meaningful groups, which would be expected to show conservation across diverse land plants. Our phylogenomic analyses of CIPKs (Figure 8; see Supplementary Figure S3 for full tree) indicated most Arabidopsis CIPKs (18 of 26) are contained within an “intronless” clade (although CIPK16 contains a single intron that is inferred to be from an intron-gain event), consistent with prior analyses by Kolukisaoglu et al. (2004). We used conifer protein sequences from this clade as queries for tBLASTn searches of Picea chromosomal sequences (available at http://congenie.org) and did not identify introns in expected locations for intron-containing CIPKs (data not shown). Based on these observations, we posit that a reverse transcription event occurred before the split of gymnosperms and angiosperms and is a conserved feature of this clade. All Physcomitrella CIPKs contain multiple introns, and none cluster with the intronless clade. Physcomitrella CIPK1—CIPK5 share high sequence similarity (83–93% pairwise); and in our analyses, they were placed with strong confidence in a clade with homologs from other mosses, indicating they are paralogs in respect to their closest seed plant homologs. This clade of moss homologs is likely orthologous (aLRT = 0.97) to three clades of CIPKs conserved across seed plants: the aforementioned intronless clade, a clade containing AtCIPK21, and a clade containing AtCIPK1 and AtCIPK17.

Arabidopsis CIPK3+CIPK26, CIPK9, and CIPK23 each represent strongly supported (aLRT = 1.0; see Supplementary Figure S3) clades that cluster with one another and contain homologs in fully sequenced angiosperm genomes and, at least for the CIPK3 + CIPK26 and CIPK23 clades, in gymnosperms. Whereas CIPK9 and CIPK23 regulate potassium transport and function in root and shoot tissues (Cheong et al., 2007; Pandey et al., 2007), CIPK3 has been implicated in abscisic acid (ABA)-dependent regulation of seed germination (Pandey et al., 2008), therefore homologs from seed plants as distantly related as gymnosperms might conceivably have a conserved regulatory role in seeds, given their strong conservation.

Physcomitrella CIPK6 and CIPK7 belong to a clade that contains Arabidopsis CIPK8 and CIPK24 and, importantly, contains all green algal CIPKs identified (Figure 9) with high confidence (aLRT = 1.0). Although Physcomitrella and Arabidopsis each contain two homologs in this clade, Physcomitrella CIPK6 and CIPK7 (72% AA pairwise identity) are the products of a gene duplication that occurred after the split between mosses and the lineage leading to vascular plants. In contrast, Arabidopsis CIPK8 and CIPK24 (60% pairwise identity) each represent a separate, strongly supported clade with orthologs in other angiosperms, implying that they derive from duplications that occurred during seed plant (most likely angiosperm) diversification. Based on our results, we posit that Physcomitrella CIPK6 and CIPK7 and Arabidopsis CIPK8 and CIPK24 (SOS2) most closely resemble the ancestral or “green algal-type” CIPK and, due to their orthology, most likely to reflect ancestral function(s) of the CBL-CIPK network.

Arabidopsis CIPK8 is believed to be a positive regulator of the low-affinity phase of the primary nitrate response and has been implicated in glucose sensing, although mechanistic details are unknown at this time (Hu et al., 2009). Arabidopsis CIPK24, the first functionally characterized CIPK, plays a critical function in sodium tolerance through CBL4(SOS3)-modulated phosphorylation of the Na⁺/H⁺ exchanger SOS1. There is substantial evidence that orthologs of CBL4 and CIPK24 in other flowering plant lineages have similar functions (Martínez-Atienza et al., 2007; Tang et al., 2010). Given the phylogenetic proximity of Arabidopsis CIPK24 to green algal CIPKs, future work will test whether green algal CIPKs, and Physcomitrella CIPK6 and CIPK7, function in Na⁺/K⁺ homeostasis or possibly more broadly regulate ion transport. It has already been established that two orthologs of SOS1 in Physcomitrella (PpSOS1 and PpSOS1b) are required for proper K⁺/Na⁺ ratios and sodium tolerance (Quintero et al., 2011). Interestingly, a 6 AA C-terminal motif of AtSOS1 that is a phosphorylation substrate of CIPK24 and a 14-3-3 protein-binding site is 100% identical to PpSOS1 and 50% identical to PpSOS1b, and the target serine is conserved in both homologs (Figure 10). Physcomitrella SOS1 has further been shown to confer enhanced NaCl tolerance when heterologously expressed in yeast, and the effect is strengthened by coexpression with Arabidopsis CBL4 and CIPK24 (Fraile-Escanciano et al., 2010). Collectively, these observations suggest that the SOS pathway is conserved across land plants and may be conserved among some green algal lineages. Functional molecular analyses of CBLs and CIPKs in early-diverging plant and algal lineages could provide core insights and clarify the increasingly complex picture of calcium-regulated abiotic stress responses in Arabidopsis and agricultural species.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.