Multiple Forms of Acanthamoeba Myosin

Extracts of Acanthamoeba castellanii were previously known to contain two myosin-like ATPases: Acanthumoeba myosin II, a double-headed enzyme of native molecular weight of about 400,000 comprised of two heavy chains of about 170,000 daltons and two pairs of light chains of about 17,500 and 17,000 daltons; and Acanthxwweba myosin I, a single-headed enzyme of native molecular weight of about 180,000. We have now shown that there are three forms of the singleheaded enzyme, all with native molecular weights of about 180,000. Acanthamoeba myosin IA has a heavy chain of 130,000 daltons and apparent light chains of 17,000 and 14,000 daltons; IB has a heavy chain of 125,000 daltons and apparent light chains of 27,000 and 14,000 daltons; IC, which has not been as highly purified as IA and IB, has a heavy chain of 130,000 daltons and apparent light chains of 20,000, 17,000, and 14,000 d&one. Enzymatically, Acanthamoeba myosins IA, IB, and IC differ from Acanthamoeba myosin II in that their (K’,EDTA)-ATPase activities are higher than their Ca”-ATPase activities, whereas Acanthamoeba myosin II has a higher Ca2’-ATPase activity, and only the three Acanthamoeba myosins I are substrates for a specific heavy chain kinase that stimulates their actin-activated Mg’-ATPase activities. Acanthamoeba myosins IA and IB have almost identical enzymatic activities. Acanthamoeba myosin IC has lower Ca2’ATPase and Mg%‘-ATPase activities, but can be converted by dissociation of the 20,000-dalton peptide into a molecule indistinguishable from Accznthamoeba myosin IA in enzymatic activity and subunit composition. The dissociated fraction that contains the 20,000d&on peptide inhibits the enzymatic activities of Acunthunwebu myosin IA. These data suggest that Acanthamoebo myosin IC is Acanthamoeba myosin IA plus a 20,000-d&on regulatory component.