Difference between subfragment-1 and heavy meromyosin in their interaction with F-actin

To elucidate the difference between subfragment-1 and heavy meromyosin in their interaction with F-actin, we used limited tryptic digestion and cross-linking with 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide. The binding of actin to subfragment-1 lowers the susceptibility of the 50K-20K junction of its heavy chain to tryptic digestion. At a molar ratio of one actin to one subfragment-1, all the sites were gradually cleaved by trypsin whereas the sites were completely protected in the presence of a 2-fold molar excess of actin over subfragment-1. In the case of heavy meromyosin, nearly half of the sites were protected completely by the presence of an equimolar amount of actin to its heads suggesting that the two heads of heavy meromyosin bound actin in a different manner. The rate of the cross-linking reaction between subfragment-1 heavy chain and actin with 1-ethyl-3-[3-(dimethylamino) propyl]carbodiimide also depended on the molar ratio of actin to subfragment-1. The rate was maximum at a molar ratio of about 5 actin to 1 subfragment-1. When heavy meromyosin was cross-linked to actin, the maximum rate was observed at a molar ratio of about 3 actin to 1 heavy meromyosin head, the level being about 60% that for subfragment-1 and actin. It was suggested that the presence of the subfragment-2 portion of heavy meromyosin caused these differences by restricting the motion of the two heads.