Modification in Skeletal ]Myosin

The rate constant of modification of a specific thiol group, SH 2, with Nethylmaleimide (NEM) has been used to estimate the conformational change in the local area containing SH2 (SH2 region) of skeletal myosin as a structural probe. The rate of Mg 2§ SH 2 modification of subfragment-1 (S-l) isozymes was regulated by Ca 2 § in the pCa range below 6.4 and was not regulated in the pCa range above 6.4. No substantial difference between S-1 containing alkali light chain, A1, (SI(A1)) and S-1 containing alkali light chain, A2, (S-I(A2)) was observed in the Ca 2 +dependent rate of SH 2 modification. Due to the presence of this Ca z § regulation in myosin (absence in S-1 isozymes) in the pCa range above 6.4, absence of 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) light chain in S-1 isozymes, and high affinity of Ca 2 + for DTNB light chain, this Ca 2 + regulation in the pCa range above 6.4 is possibly related to the Ca 2+ binding to DTNB light chain. F-Actin, which is entirely free from tropomyosin and troponin, enhanced the rate of Mg2+-ATP-induced SH 2 modification of S-1 isozymes equally and of myosin, and reduced the Ca 2 + sensitivity with an increase in F-actin concentration.