Effects of SH1 and SH2 modifications on myosin similarities and differences

The properties of myosin modified at the SH2 group (Cys-697) were studied a nd compared with the previously reported properties of myosin modified at t he SH1 group (Cys-707). 4-[N-[(iodoacetoxy)ethyl]-N methylamino]-7-nitroben z-2-oxa-1,3-diazole (IANBD) was used for selective modification of the SH2 group on myosin. SH2-labeled heavy meromyosin (SH2-HMM), similar to SH1-lab eled HMM (SH1-HMM), did not propel actin filaments in the in vitro motility assays. SH1- and SH2-HMM produced similar amounts of load in the mixtures with unmodified HMM; the sliding speed of actin filaments gradually decreas ed with an increase in the fraction of either one of the modified HMMs in t he mixture. In analogy to SH1-labeled myosin subfragment 1 (SH1-S1), SH2-la beled S1 (SH2-S1) activated regulated actin in the in vitro motility assays . SH2 modification inhibited Mg-ATPase of S1 and its activation by actin. T he weak binding of S1 to actin was unaffected whereas the strong binding wa s weakened by SH2 modification. Overall, our results demonstrate similar be havior of SH1- and SH2-modified myosin heads in the in vitro motility assay s despite some differences in their enzymatic properties. The effects of th ese modifications are ascribed to the location of the SH1-SH2 helix relativ e to other functional centers of S1.