Bundling of myosin subfragment-1-decorated actin filaments.

We have reported previously that rabbit skeletal myosin subfragment-1 (S-1) assembles actin filaments into bundles. The rate of this reaction can be estimated roughly from the initial rate (Vo) of the accompanying turbidity increase ("super-opalescence") of the acto-S-1 solution. Vo is a function of the molar ratio (r) of S-1 to actin, with a peak at r = 1/6 to 1/7 and minimum around r = 1. In the present paper we report a different type of opalescence (we call it "hyper-opalescence") of acto-S-1 solutions, which also resulted from bundle formation. Adjacent filaments in the bundles had a distance of approximately 180 A. Hyper-opalescence occurred at r approximately equal to 1 when KCOOCH3 was used instead of KCl. By comparing the effects of ADP, epsilon-ADP, tropomyosin or ionic strength upon the super- and hyper-opalescence, we concluded that the two types of S-1-induced actin bundling had different molecular mechanisms. The hyper-opalescence type of bundling seemed to be induced by S-1, which was not complexed with actin in the manner of conventional rigor binding. The presence of the regulatory light chain did not affect hyper-opalescence (or super-opalescence), since there were no significant differences between papain S-1 and chymotryptic S-1 with respect to these phenomena.