Determinants for substrate phosphorylation by Dictyostelium myosin II heavy chain kinases A and B and eukaryotic elongation factor-2 kinase

The α kinases are a widespread family of atypical protein kinases characterized by a novel type of catalytic domain. In this paper the peptide substrate recognition motifs for three α kinases, Dictyostelium discoideum myosin heavy chain kinase (MHCK) A and MHCK B and mammalian eukaryotic elongation factor-2 kinase (eEF-2K), were characterized by incorporating amino acid substitutions into a previously identified MHCK A peptide substrate (YAYDTRYRR) (Luo X. et al. (2001) J. Biol. Chem. 276, 17836–43). A lysine or arginine in the P+1 position on the C-terminal side of the phosphoacceptor threonine (P site) was found to be critical for peptide substrate recognition by MHCK A, MHCK B and eEF-2K. Phosphorylation by MHCK B was further enhanced 8-fold by a basic residue in the P+2 position whereas phosphorylation by MHCK A was enhanced 2- to 4-fold by basic residues in the P+2, P+3 and P+4 positions. eEF-2K required basic residues in both the P+1 and P+3 positions to recognize peptide substrates. eEF-2K, like MHCK A and MHCK B, exhibited a strong preference for threonine as the phosphoacceptor amino acid. In contrast, the Dictyostelium VwkA and mammalian TRPM7 α kinases phosphorylated both threonine and serine residues. The results, together with a phylogenetic analysis of the α kinase catalytic domain, support the view that the metazoan eEF-2Ks and the Dictyostelium MHCKs form a distinct subgroup of α kinases with conserved properties.