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 alpha-kinase family, phosphorylates sites in the myosin II tail that block filament assembly. Here we show that the catalytic activity of A-CAT, the alpha-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 Thr(825) using shrimp alkaline phosphatase decreased A-CAT activity. The activity of a truncated A-CAT lacking Thr(825) could be rescued by Pi, phosphothreonine, and a phosphorylated peptide, but not by threonine, glutamic acid, aspartic acid, or an unphosphorylated peptide. These results focused attention on a Pi-binding pocket located in the C-terminal lobe of A-CAT. Mutational analysis demonstrated that the P-1-pocket was essential for A-CAT activity. Based on these results, it is proposed that autophosphorylation of Thr(825) activates ACAT by providing a covalently tethered ligand for the Pi-pocket. Ab initio modeling studies using the Rosetta FloppyTail and Flex-PepDock protocols showed that it is feasible for the phosphorylated Thr(825) to dock intramolecularly into the Pi-pocket. Allosteric activation is predicted to involve a conformational change in Arg(734), which bridges the bound Pi to Asp(762) 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.