Some properties of uterus acomyosin and myofilaments

Myosin heavy chain isoforms and smooth muscle function

Using isoform specific antibodies we have verified the presence of two distinct muscle type myosin heavy chain isoforms in rat uterine muscle. We have shown that an endogenous protease can cleave a small 4 kDa region from the C-terminal of the SM1 isoform which generates a pSM1 species which comigrates with the SM2 isoform on low density SDS gels. While this cleavage can complicate isoform identification, more importantly, this cleavage was associated with a substantial increase in the actomyosin ATPase. Thus we have identified a domain at the C-terminal which may be involved in regulation of the ATPase activity. Interestingly, it is at this C-terminal, tail region of the smooth muscle myosin molecule where the only known isoform specific sequence differences are located. In skinned smooth muscle fibers of rat uterine muscle, we have also shown that differences in myosin heavy chain distribution, induced by beta-estradiol treatment of ovariectomized rats, are correlated with changes in unloaded shortening velocity. Thus our work suggests that the functional significance of myosin heavy chain isoforms in smooth muscle may be similar to that observed in striated muscle.

Preparation and properties of vertebrate smooth-muscle myofibrils and actomyosin

A new technique for obtaining a myofibril-like preparation from vertebrate smooth muscle has been developed. An actomyosin can be readily extracted from these myofibrils at low ionic strength and in yields 20 times as high as previously reported. The protein composition of all preparations has been monitored using dodecylsulfate-gel electrophoresis. By this method smooth muscle actomyosin showed primarily only the major proteins, myosin, actin and tropomyosin, while the myofibrils contained, additionally, three new proteins not previously described with polypeptide chain weights of 60000, 110000 and 130000. The ATPase activities of both the myofibrils and actomyosin preparations are considerably higher than previously described for vertebrate smooth muscle. They are sensitive to micromolar Ca2+ ion concentrations to the same degree as comparable skeletal and cardiac muscle preparations, even though troponin-like proteins could not be identified in these smooth muscle preparations. From the latter observation and the presence of Ca2+-sensitivity in tropomyosin-free actomyosin it is suggested that this calcium sensitivity is, as in some invertebrate muscles, a property of the myosin molecule.

An electron microscope study of the influence of divalent ions on myosin filament formation in chicken gizzard extracts and homogenates

Myosin as well as actin filaments could be seen in negatively stained preparations of fresh chicken gizzard homogenized in buffered KCI (I=0.12, pH 6.85), in a 1 : 1 ratio. Myosin filaments were also present when homogenates were diluted (1 : 9) with solutions containing additional Mg(++) and ATP provided naturally occurring traces of Ca(++) had not been chelated with EGTA. There were no myosin filaments when Mg(++), ATP, Ca(++) or Ca(++) +ATP were added respectively to the diluent. It is suggested that in vivo in relaxed muscles myosin is present in dimers, and only aggregates into filaments at the onset of contraction when Ca(++) are released.