IDENTIFICATION OF A NOVEL SMOOTH-MUSCLE MYOSIN HEAVY-CHAIN CDNA - ISOFORM DIVERSITY IN THE S1 HEAD REGION

Smooth muscle myosin heavy chain (SMHC) isoforms, SM1 and SM2, are the products of alternative splicing from a single gene (P. Babij and M. Periasamy. J. Mol. Biol. 210: 673-679, 1989). We have previously shown that this splicing occurs at the 3'-end of the mRNA, resulting in pro teins that differ at the carboxy-terminal (R. Nagai, M. Kuro-o, P. Bab ij, and M. Periasamy. J. Biol. Chem. 264: 9734-9737, 1989). In the pre sent study we demonstrate that additional SMHC isoform diversity occur s in the globular head region by isolating and characterizing two dist inct rat SMHC cDNA (SMHC-11 = SM1B and SMHC-5 = SM1A). Sequence compar ison of the two clones reveals that they are completely identical in t heir coding regions, except at the region encoding the 25/50 kDa junct ion of the myosin head, where the SM1B isoform contains an additional seven amino acids. This divergent region is located adjacent to the Mg 2+-ATPase site, and differences in this region may be of functional im portance. Ribonuclease protection analysis demonstrates that the corre sponding SM1B and SM1A mRNA messages are coexpressed in all smooth mus cle tissues; however, the proportion of the two mRNA present differs s ignificantly between tissues. The SM1A-type mRNA predominates in most smooth muscle tissues, with the exception of intestine and urinary bla dder, which contain greater proportions of the SM1B message. The diffe rential distribution of these two isoforms may provide important clues toward understanding differences in smooth muscle contractile propert ies.