Myosin

Abstract

There is fairly general agreement that myosin isolated from rabbit skeletal muscle has a molecular weight of about 500,000. The higher values that have been reported apparently reflect protein aggregation related to the method of preparation. On the basis of present evidence, the myosin molecule has an elongate helical core of two f subunits (average weight about 215,000) that extend into a globular head region containing three g subunits (average weight about 20,000). Myosin may be dissociated into subunits by a number of methods. In 5 M guanidine, the myosin molecule is dissociated into f and g subunits, while at pH above 10, the g subunits are dissociated from the intact fibrous core of myosin. The dissociation of g subunits at pH 10 is accompanied by the loss of both ATPase activity and actin-binding capacity; however, the exact biological significance of the g subunits is presently uncertain. In preliminary studies, the f subunits appear to contain the sulfhydryl residues currently implicated in myosin ATPase, and there is some indication of allosteric regulation of enzymic activity.

The Identification of Myosin in Rabbit Hepatocytes

“…k 5000, respectively. These values are in agreement with present estimates of the subunit sizes of the muscle proteins [37,38].…”

Studies on the Role of Light and Heavy Chains in Myosin Adenosine Triphosphatase

“…In previous ultracentrifugal studies on rabbit skeletal myosin, we have reported that the light chains have a weight average tool wt of 20,000 and comprise approximately 12 ~ of the total myosin, the intact molecule having a mol wt of 470,000 (Gershman et al, 1966(Gershman et al, , 1969Dreizen et al, 1967). This evidence would imply the presence of three light chains (on the average) per myosin molecule.…”

“…In general those denaturing conditions which result in complete dissociation of light chains from myosin also lead to irreversible inactivation of myosin ATPase. For example, myosin ATPase is irreversibly denatured on alkaline treatment above pH 10, over the same range of pH where light chains are dissociated from myosin (Gershman et al, 1966(Gershman et al, , 1969Dreizen et al, 1967). A close relationship between subunit dissociation and irreversible loss of ATPase has also been found in a series of experiments on myosin in concentrated salt solutions at pH 7 and 4~ (Gershman et al, 1968;Dreizen and Gershman, 1970).…”

“…After long controversy, there now seems to be general agreement that myosin (470,000 tool wt) comprises an axial core of two heavy polypeptide chains (205,000 mol wt) that terminate in a globular region containing approximately three light chains of average tool wt about 20,000 (Gershlnan et al, 1966(Gershlnan et al, , 1969Dreizen et al, 1967;Locker and Hagyard, 1967a, b;Gazith et al, 1970;Weeds and Lowey, 1971). The globular head of myosin appears to contain symmetric halves of subfragment-1 (Slayter and Lowey, 1967;Trotta et al, 1968;, each of which contains one light chain and a remnant of one heavy chain (Trotta et al, 1968).…”

“…The functional role of light and heavy polypeptide chains in the ATPase activity of myosin has been explored in studies on the denaturation of myosin under conditions like heat, alkaline pH, and concentrated salt solutions (Gershman et al, 1968(Gershman et al, , 1969Dreizen et al, 1967;Dreizen and Gershman, 1970), and on the properties of reconstituted myosin or its component subunits following subunit fractionation under denaturing conditions (Gershman et al, 1969;Stracher, 1969;Dreizen and Gershman, 1970). In general those denaturing conditions which result in complete dissociation of light chains from myosin also lead to irreversible inactivation of myosin ATPase.…”