REGULATORY LIGHT-CHAINS IN MYOSINS

Scallop [Aequipecten irradians, Placopecten magellanicus, Mercenaria mercenaria, Spisula solidissima, Pecten maximus] myosin molecules contain 2 mol of regulatory light chains and 2 mol of light chains with unknown function. Removal of 1 of the regulatory light chains by treatment with EDTA is accompanied by the complete loss of the Ca dependence of the actin-activated ATPase activity and by the loss of 1 of the 2 Ca binding sites on the intact molecule. Such desensitized preparations recombine with 1 mol of regulatory light chain and regain Ca regulation and Ca binding. The 2nd regulatory light chain may be selectively obtained from EDTA-treated scallop muscles by treatment with the Ellman reagent (5,5''-dithiobis(2-nitrobenzoic acid [DTNB]): treatment with this reagent, however, leads to an irreversible loss of ATPase activity. Light chains obtained by treatment with EDTA and then DTNB are identical in composition and function. A different light chain fraction obtained by subsequent treatment with guanidine-HCl does not bind to desensitized or intact myofibrils and has no effect on ATPase activity. Regulatory light chains which bind to desensitized scallop myofibrils with high affinity and restore Ca control were found in a number of molluscan and vertebrate myosins, including Mercenaria, Spisula, squid [Loligo forbesi], lobster tail, beef heart, chicken gizzard, frog and rabbit. Although these myosins all have a similar subunit structure and contain about 2 mol of regulatory light chain, only scallop myosin or myofibrils can be desensitized by treatment with EDTA. There appear to be 2 classes of regulatory light chains. The regulatory light chain of mollusks and of vertebrate smooth muscles restore full Ca binding and resensitize purified scallop myosin. The regulatory light chains from vertebrate striated, cardiac and the fast decapod muscles have no effect on Ca binding and do not resensitize purified scallop myosin unless the myosin is complexed with actin. The latter class of light chains is found in muscles where in vitro functional tests failed to detect myosin-linked regulation.