Alternative Splicing of Smooth Muscle Heavy Chain Carboxyl Terminal

Alternative Splicing of Smooth Muscle Heavy Chain Carboxyl Terminal 

It is now clear from studies analyzing both the smooth muscle HC gene and its products that the pre-mRNA undergoes alternative splicing in at least two different locations. The first is located at the 3 end of the coding sequence and results in HCs with two different MWs, 204,000 and 200,000 on SDS-PAGE (see Fig. 2) (Nagai et al., 1989 Babij and Periasamy, 1989). 

The HC204 HC200 ratio increased from 2.1 1 in nonpregnant rats to 2.6 1 in the pregnant rat. The uterine values contrasted to that found in the rat portal vein, 

Because of the dimeric nature of the myosin molecule, the presence of both 204- and 200-kDa isoforms in smooth muscle cells raises the question of whether individual myosin molecules are composed of homodimers of 204- and 200-kDa HCs, heterodimers, or a mixture of homodimers and heterodimers. The observation that some smooth muscles contained equal amounts of HC204 and HC200 was consistent with the possibility that they were composed solely of heterodimers. However, the subsequent finding of non-stoichiometric ratios for the two heavy chains is consistent with the presence of homodimers only, or with a mixture of homodimers and heterodimers. 

Using a different, more indirect method, Tsao and Eddinger (1993) concluded that porcine and rabbit smooth muscle myosin molecules from a variety of tissues do contain significant amounts of heterodimer 

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