Autophosphorylation activates Dictyostelium myosin II heavy chain kinase A by providing a ligand for an allosteric binding site in the α-kinase domain

Dictyostelium discoideum myosin II heavy chain kinase A (MHCK A), a member of the atypical α-kinase family, phosphorylates sites in the myosin II tail that block filament assembly. Here we show that the catalytic activity of A-CAT, the α-kinase domain of MHCKA (residues 552-841), is severely inhibited by the removal of a disordered C-terminal tail sequence (C-tail; residues 806-841). The key residue in the C-tail was identified as Thr ⁸²⁵, which was found to be constitutively autophosphorylated. Dephosphorylation of Thr⁸²⁵ using shrimp alkaline phosphatase decreased A-CAT activity. The activity of a truncated A-CAT lacking Thr ⁸²⁵ could be rescued by P_i, phosphothreonine, and a phosphorylated peptide, but not by threonine, glutamic acid, aspartic acid, or an unphosphorylated peptide. These results focused attention on a P _i-binding pocket located in the C-terminal lobe of A-CAT. Mutational analysis demonstrated that the P_i-pocket was essential for A-CAT activity. Based on these results, it is proposed that autophosphorylation of Thr⁸²⁵ activates ACAT by providing a covalently tethered ligand for the P_i-pocket. Ab initio modeling studies using the Rosetta FloppyTail and Flex-PepDock protocols showed that it is feasible for the phosphorylated Thr⁸²⁵ to dock intramolecularly into the P _i-pocket. Allosteric activation is predicted to involve a conformational change in Arg⁷³⁴, which bridges the bound P _i to Asp⁷⁶² in a key active site loop. Sequence alignments indicate that a comparable regulatory mechanism is likely to be conserved in Dictyostelium MHCKB-D and metazoan eukaryotic elongation factor-2 kinases.