Effects of SH1 and SH2 modifications on myosin: similarities and differences.

The properties of myosin modified at the SH2 group (Cys-697) were studied and compared with the previously reported properties of myosin modified at the SH1 group (Cys-707). 4-[N-[(iodoacetoxy)ethyl]-N methylamino]-7-nitrobenz-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-labeled 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 decreased with an increase in the fraction of either one of the modified HMMs in the mixture. In analogy to SH1-labeled myosin subfragment 1 (SH1-S1), SH2-labeled S1 (SH2-S1) activated regulated actin in the in vitro motility assays. SH2 modification inhibited Mg-ATPase of S1 and its activation by actin. The weak binding of S1 to actin was unaffected whereas the strong binding was weakened by SH2 modification. Overall, our results demonstrate similar behavior of SH1- and SH2-modified myosin heads in the in vitro motility assays despite some differences in their enzymatic properties. The effects of these modifications are ascribed to the location of the SH1-SH2 helix relative to other functional centers of S1.