Fluorescence studies on the nucleotide- and Ca2+-binding domains of molluscan myosin.

The effects of nucleotides and Ca2+ on the intrinsic tryptophan fluorescence of molluscan myosin and its proteolytic fragments were studied. By using these proteins from the scallop, Pecten maximus, the existence of two distinct tryptophan-containing domains was established, which respond independently to ATP and Ca2+-specific binding. The latter is located in the 'neck' region of the myosin, which constitutes the regulatory domain. Subfragment 1, lacking the regulatory domain, responded only to ATP binding. On the other hand a tryptic fragment comprising the regulatory domain responded only to Ca2+ binding. Subfragment 1, containing the regulatory domain, responded to both ATP and Ca2+, but its ATPase activity was Ca2+-insensitive. By contrast, the ATPase activity of HMM was Ca2+-sensitive. Increasing the ionic strength had a detrimental effect on Ca2+-sensitivity, and fluorescence studies on solubilized myosin were therefore of limited value. Myosin and its fragments from other molluscan species which were investigated produced similar changes to those of Pectan maximus.