Binding of monovalent and divalent myosin fragments onto sites on actin

Abstract

THERE has been considerable interest recently in the comparative and competitive equilibrium binding of S1 (sub-fragment 1) and HMM (heavy meromyosin) on F-actin, all in solution. S1 is a single myosin ‘head’, HMM has two heads, and F-actin consists of two strands of polymerised actin monomers¹. One myosin head can bind to one actin monomer. This type of one-dimensional binding can be studied using the well-known matrix method in statistical mechanics. The general method and a number of examples were discussed previously². It is necessary, in using this method, to manipulate the largest root of a polynomial. This is simple if the polynomial is a quadratic but more difficult otherwise. The problem can be handled by computer methods in the more complicated cases. The purpose of this note is to modify one of the earlier examples² so as to allow a more general analysis of S1 and HMM binding than previously possible. The binding isotherms derived below are so easy to calculate, illustrative plots are left to those readers interested in particular cases. (See refs 3 and 4 for further background.)