Projectin-thin filament interactions and modulation of the sensitivity of the actomyosin ATPase to calcium by projectin kinase

The insect muscle protein projectin (900 kDa) belongs to a novel family of cytoskeleton-associated protein kinases (titin, twitchin, and projectin) that are members of the immunoglobulin superfamily. The functions of these kinases are still unknown although recent data suggest a role in modulating muscle activity and generating passive elasticity. An important question is what are the in vivo substrates for these enzymes. We found a thin filament-associated 30 kDa protein that acts as an in vitro substrate for projectin kinase from Locusta migratoria. However, we did not find activators for projectin kinase. Neither calcium, calcium with calmodulin, nor cAMP activated the in vitro activity of projectin kinase. Binding studies revealed a strong interaction between projectin and thin filaments comparable with that of the projectin-myosin interaction. That an interaction might be possible in vivo is suggested by immunological studies showing that projectin is attached to the surface of myosin filaments. Since the molecular weights indicate that the 30 kDa protein might be troponin I, which is known to play a central role in modulating cardiac contractile activity, we studied whether phosphorylation of this protein by projectin changes the calcium sensitivity of the actomyosin ATPase. We found a significant increase in the calcium sensitivity. Thus, our results indicate the existence of a novel mechanism of regulation of muscle activity by a cytoskeleton-associated kinase.