COOPERATIVITY IN F-ACTIN - CHEMICAL MODIFICATIONS OF ACTIN MONOMERS AFFECT THE FUNCTIONAL INTERACTIONS OF MYOSIN WITH UNMODIFIED MONOMERS IN THE SAME ACTIN FILAMENT

We have chemically modified a fraction of the monomers in actin filame nts, and then measured the effects on the functional interaction of my osin with unmodified monomers within the same filament. Two modificati ons were used: (a) covalent attachment of various amounts of myosin su bfragment-1 (S1) with the bifunctional reagent disuccinimidyl suberate and (b) copolymerization of unmodified actin monomers with monomers c ross-linked internally with 1-ethyl-3-(dimethylaminopropyl)-carbodiimi de. Each of these modifications abolished the interaction of the modif ied monomers with myosin, so the remaining interactions were exclusive ly with unmodified monomers. The two modifications had similar effects on the interaction of actin with myosin in solution: decreased affini ty of myosin heads for unmodified actin monomers, without a change in the V(max) of actin-activated myosin ATPase activity. However, modific ation (b) produced much greater inhibition of actin sliding on a myosi n-coated surface, as measured by an in vitro motility assay. These res ults provide insight into the functional consequences of cooperative i nteractions within the actin filament.