Identification of a Novel Phosphorylation Motif for CDPKs: Phosphorylation of Synthetic Peptides Lacking Basic Residues at P-3/P-4

The Ca2+-dependent protein kinases (CDPKs) are members of a large subfamily of protein kinases in plants that have been implicated in the control of numerous aspects of plant growth and development. One known substrate of the CDPKs is the ER-located ACA2 calcium pump, which is regulated by phosphorylation of Ser45. In the present study, a synthetic peptide based on the known regulatory phosphorylation site (RRFRFTANLS45KRYEA) was efficiently phosphorylated in vitro by CDPKs but not a plant SNF1-related protein kinase. Phosphorylation of the Ser45-ACA2 peptide was surprising because the sequence lacks basic residues at P-3/P-4 (relative to the phosphorylated Ser at position P) that are considered to be essential recognition elements for CDPKs. We demonstrate that phosphorylation of the Ser45-ACA2 peptide is dependent on the cluster of basic residues found N-terminal (P-6 to P-9) as well as C-terminal (P + 1/P + 2) to the phosphorylated Ser. The results establish a new general phosphorylation motif for CDPKs: [Basic-Basic-X-Basic]-ϕ-X(4)-S/T-X-Basic (where ϕ is a hydrophobic residue). The motif predicts a number of new phosphorylation sites in plant proteins. Evidence is presented that the novel motif may explain the phosphorylation by CDPKs of Ser271 in the aquaporin PM28A.