CORRELATION BETWEEN CALPONIN AND MYOSIN SUBFRAGMENT-1 BINDING TO F-ACTIN AND ATPASE INHIBITION

Calponin is a thin-filament-associated protein that has been implicate d in the regulation of smooth-muscle contractility. It binds to F-acti n and inhibits the MgATPase activity of actomyosin. In the present wor k we have examined the effect of recombinant chicken gizzard alpha-cal ponin (R alpha CaP) on the binding of rabbit skeletal-muscle myosin su bfragment 1 (S1) to F-actin and on the inhibition of its actin-activat ed MgATPase. We have found that binding of one R alpha CaP molecule to every three to four actin monomers is sufficient for maximal inhibiti on of acto-S1 ATPase. At this R alpha CaP/actin ratio R alpha CaP does not interfere with S1 binding to F-actin. At higher concentrations, R alpha CaP displaces S1 from F-actin and a 1:1 R alpha CaP-actin monom er complex is formed. R alpha CaP is also able to displace troponin I from its complex with F-actin which may reflect the amino acid sequenc e similarity between R alpha CaP and troponin I in their actin-binding regions.