Kinase-related protein: a smooth muscle myosin-binding protein

Kinase-related protein (telokin) is a small myosin-binding protein which has recently been discovered in smooth muscle. The KRP messenger RNA is transcribed from within the myosin light chain kinase (MLCK) gene, a rare example in vertebrates of two proteins coded for by a single gene. Owing to a separate transcription unit and a common reading frame, kinase-related protein is expressed as an independent protein which consists of the C-terminal 156 amino acids of the kinase. It binds specifically to dephosphorylated smooth muscle myosin and inhibits myosin phosphorylation by MLCK in vitro, suggesting that it might modify the rate of myosin activation and consequently the rate of tension development in the muscle. KRP also stabilizes an extended conformation of dephosphorylated myosin which can polymerize, and thereby stabilizes myosin in the filamentous state against the dissociating effect of ATP. Thus, kinase-related protein may have a function in regulating the assembly of myosin filaments into the contractile apparatus in the cell.