A carrot cDNA encoding an atypical protein kinase homologous to plant calcium-dependent protein kinases

Abstract

Calcium-dependent protein kinases (CDPKs) in plants typically contain a C-terminal calmodulin-like domain with four EF-hand calcium-binding motifs. We have isolated a carrot somatic embryo cDNA clone which encodes a new, divergent isoform of this family, designated CRK (CDPK-related kinase). The catalytic domain of CRK shares a high degree of homology with the catalytic domains of plant CDPKs (53.5% average identity with its two closest phylogenetic relatives, CDPK431 (carrot) and AK1 (Arabidopsis). However, the C-terminal domain of CRK bears significantly less homology to calmodulin (22.0% identity to barley calmodulin) than other plant CDPKs (38.0% average identity between barley calmodulin and the C-terminal domains of CDPK431 and AK1). This degeneracy also involves the EF-hand motifs of CRK, which have diverged to varying extents. The predicted structure of CRK also contains an extended N-terminal domain 145 amino acids in length possessing a consensus N-myristoylation signal. CRK transcripts are most abundant in somatic embryos, with lesser accumulations in flowers and leaves and lowest levels in roots. Homologous genomic DNA sequences that hybridize with CRK cDNA but not with a carrot CDPK probe have been detected in a variety of higher plant taxa, including monocotyledonous species, suggesting that this CDPK-related kinase is widely conserved among angiosperms.

The Arabidopsis CDPK-SnRK Superfamily of Protein Kinases

“…The CRK nomenclature was proposed by Lindzen and Choi (1995). These kinases have so far only been identified in angiosperms.…”

Inactivation of Plasma Membrane–Localized CDPK-RELATED KINASE5 Decelerates PIN2 Exocytosis and Root Gravitropic Response in Arabidopsis

“…All so far known CRKs, including CRK5, share a conserved C-terminal calmodulin (CaM) binding domain, which overlaps with the kinase autoinhibitory domain (Zhang et al, 2002). Selective CaM binding in the presence of Ca 2+ simulates autophosphorylation but leads only to marginal increase of substrate phosphorylation by CRKs, which are equally active without CaM in the presence of either Ca 2+ or the Ca 2+ chelator EGTA (Lindzen and Choi, 1995;Furumoto et al, 1996;Lu et al, 1996;Wang et al, 2001;Zhang et al, 2002;Hua et al, 2003Hua et al, , 2004Wang et al, 2004;Leclercq et al, 2005).…”

Plant calcium‐dependent protein kinase‐related kinases (CRKs) do not require calcium for their activities

“…Evidently, our maize cDNAs encoded members of the protein kinase superfamily. After their amino acid sequences had been completely deduced, it turned out that our clones were highly homologous to the recently cloned carrot CRK [4]. The overall amino acid identity of ZmCRK3 with carrot CRK was 58%.…”

“…Another phylogenetic tree based on the amino acid sequences of entire polypeptides showed a similar pattern (data not shown). The carrot CRK was once reported to be more similar to CDPKs than to Plasmodium putative CDPK [4]. However, the Plasmodium enzyme is highly degenerate so that it is not considered to be a typical member of the CDPK subfamily [11].…”

“…A maize (Zea mays H84) leaf cDNA library in )~gtl0 [4] was screened with a mixture of ;~2p_ labeled cDNA fragments as probes, under low stringency conditions, as described [5]. A faintly positive clone, designated as ZmPK2, was found.…”