Thiols of myosin. VI. Effect of ADP on the reactivity of thiol groups other than S2.

Experiments were carried out to explore the location of thiol groups which became less reactive when S2 became more reactive in the presence of ADP. Myosin was first treated with N-ethylmaleimide to block S1 and then treated with a fluorogenic thiol reagent, N-(7-dimethyl-amino-4-methylcoumarinyl) maleimide (DACM), in the presence or absence of ADP. From the distribution of DACM in the two kinds of DACM-treated myosin, it was found that thiol groups of all the light chains and of the 50 K fragment of subfragment-1 heavy chain became less reactive to DACM in the presence of ADP. Since it is known that the ADP binding site and S2 are located in the 26 K and 21 K fragments of subfragment-1 heavy chain, respectively, our results suggest that the 50 K, 26 K, and 21 K fragments of subfragment-1 heavy chain and light chains are closely connected with each other in the subfragment-1.