Substrate specificity and inhibitor sensitivity of Ca²⁺/S100-dependent twitchin kinases

Myosin-associated giant protein kinases of the titin/twitchin-like superfamily have previously been implicated in the regulation of muscle function, based on genetic and physiological studies. We find that recombinant constitutively active Caenorhabditis elegans and Aplysia twitchin kinase fragments differ in their catalytic activities and peptide-substrate specificities, as well as in their sensitivities to the naphthalene sulfonamide inhibitors 1-(5-chloronaphthalenesulfonyl)-1 H-hexahydro-1,4-diazepine (ML-7) and 1-(5-iodonaphthalenesulfonyl)-1 H-hexahydro-1,4-diazepine (ML-9). The constitutively active Aplysia twitchin kinase fragment has a remarkably high activity (V_max > 100 μmol · min 1 · mg ¹) towards some substrate peptides. The autoinhibited forms of these twitchin kinases can be activated in a Ca²⁺-dependent manner by the dimeric form of the S100A1 protein (S100A1₂). The twitchin kinase S100A1₂-binding site can also bind Ca²⁺/calmodulin but neither kinase is activated by calmodulin. The data provide a functional basis for the ongoing crystallographic study of twitchin kinase fragments.