The functional significance of structural isoforms of myosin heavy chain in smooth muscle

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" The functional significance of structural isoforms of myosin heavy chain in smooth muscle "
T. E. Hewett A. F. P. Martin, Richard J.

Document Type	:	Latin Dissertation
Language of Document	:	English
Record Number	:	1112613
Doc. No	:	TLpq303831492
Main Entry	:	A. F. P. Martin, Richard J.
	:	T. E. Hewett
Title & Author	:	The functional significance of structural isoforms of myosin heavy chain in smooth muscle\ T. E. HewettA. F. P. Martin, Richard J.
College	:	University of Cincinnati
Date	:	1990
student score	:	1990
Degree	:	Ph.D.
Page No	:	148
Abstract	:	Two myosin heavy chain (MHC) isoforms, SM1 (204 kDa) and SM2 (200 kDa), have been identified in smooth muscle. How these two smooth muscle isoforms differ functionally is not known. It is well known that isoforms of myosin from striated muscle are functionally different. We demonstrate that skinned myometrial fibers from ovariectomized and estrogen-treated rats differ significantly in MHC composition and functional performance. Unloaded velocity of shortening increases subsequent to estrogen treatment and correlates significantly with an increased relative proportion of SM1 in these fibers. This increased velocity of shortening likely reflects an increased enzymatic activity of SM1 relative to SM2. Limited proteolysis of SM1 generates a species (pSM1) which comigrates on SDS-PAGE with SM2. pSM1 possesses a Ca-ATPase activity which is markedly increased relative to that of the intact isoforms, and unlike that of the intact isoforms, is independent of the ionic strength of the assay buffer. A similar proteolyzed product of SM1 can be generated by usd\alphausd-chymotryptic cleavage of gizzard myosin. As a result of proteolysis of phosphorylated turkey gizzard myosin by chymotrypsin the following changes are observed: (a) the Mg-dependence of actin-activated ATPase of digested phosphorylated myosin is altered; (b) the KCl-dependence of Mg-ATPase of the digested myosin, particularly the phosphorylated form, demonstrates a leftward shift, i.e. higher activity at lower ionic strength; (c) activation of the actin-activated ATPase above values observed for fully phosphorylated myosin at high ionic strength (>0.25 M). These data suggest that a protein domain exists on SM1 which inhibits ATPase activity. In order to localize the site of this domain, we developed antibodies specific to the C-terminus of either SM1 or SM2 which we employed to unequivocally demonstrate that the proteolytically cleaved domain arises from the C-terminus of the SM1 molecule from both rat uterus and turkey gizzard. These results indicate that C-terminal regulation of N-terminal ATPase activity may occur in smooth muscle as it does in nonmuscle. We postulate that this regulation may occur at the level of thick filament formation. In conclusion, a small C-terminal sequence difference may be the sole basis for differences in the enzymatic activities of these two heavy chain isoforms.
Subject	:	Anatomy physiology
	:	Anatomy physiology
	:	Animals
	:	Biological sciences
	:	Biophysics
	:	myosin heavy chain isoforms