title: Interaction of myosin subfragment-1 with actin. III. Effect of cleavage of the subfragment-1 heavy chain on its interaction with actin.
abstract: To determine the reason why the Mg2+-ATPase activity of subfragment-1 prepared with chymotrypsin was activated more by actin than that of subfragment-1 prepared with trypsin was and the reason why the former could enhance the polymerization of actin and the latter could not, we digested subfragment-1, prepared with chymotrypsin, with trypsin and examined the actin activated Mg2+-ATPase activity and the ability to polymerize actin. It was found that cleavage of the heavy chain decreased the actin activated Mg2+-ATPase activity of subfragment-1 prepared with chymotrypsin but did not affect its ability to polymerize actin. Trypsin attacked the subfragment-1 heavy chain at two sites and produced 26 K, 50 K, and 21 K fragments. From the comparison of the time course of tryptic digestion with that of the decrease in actin activation, it was deduced that cleavage of the 50 K-21 K junction was mainly responsible for the decrease in actin activation. We also measured the length and the amount of F-actin polymerized by the addition of different amounts of subfragment-1. It was found that the amount of F-actin increased with the increase in the amount of subfragment-1 added and that the length of F-actin also increased though slightly. We concluded from the results that subfragment-1 enhanced the polymerization not only by facilitating the nucleus formation but also by strengthening the bond between actin monomers in forming F-actin.
mesh_terms: D000199:Actins / Q000378:metabolism*; D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D007700:Kinetics; D046911:Macromolecular Substances; D008274:Magnesium / Q000494:pharmacology; D008854:Microscopy, Electron; D008970:Molecular Weight; D009132:Muscles / Q000201:enzymology*; D009218:Myosins / Q000378:metabolism*; D011817:Rabbits; D014357:Trypsin
chemical_list: D000199:Actins; D046911:Macromolecular Substances; D014357:Trypsin; D000251:Adenosine Triphosphatases; D009218:Myosins; D000252:Calcium-Transporting ATPases; D008274:Magnesium