MYOSIN HEAVY-CHAIN ISOFORM EXPRESSION IN RAT SMOOTH-MUSCLE DEVELOPMENT

Smooth muscle myosin heavy chains (MHCs), the motor proteins that powe r smooth muscle contraction, are produced by alternative splicing from a single gene. The smooth muscle MHC gene is capable of producing,fou r isoforms by utilizing alternative splice sites located at the region s encoding the carboxy terminus and the junction of the 25- and 50-kDa tryptic peptides. These four isoforms, SM1A, SM1B, SM2A, and SM2B, ar e a combination of one of two heavy chains containing different carbox y-terminal tails (1 or 2) without (A) or with (B) an additional motif in the myosin head. In the present study, using RNA analysis and isofo rm-specific antibodies, we demonstrate the expression patterns of MHC isoforms during development in rat smooth muscle tissues. RNase protec tion analysis indicates that the mRNAs for SMA and SMB isoforms, which differ by a 21-nucleotide insertion in the region encoding the S1 hea d region of the myosin molecule, are differentially expressed during d evelopment in a highly tissue-specific manner. Smooth muscle MHC trans cripts are first detectable in developing rat smooth muscle tissues at 17 days of fetal development. The SMB mRNA is shown to be expressed i n smooth muscle from fetal bladder, intestine, and stomach and from ne onatal aorta; however, it is not expressed in cultured smooth muscle c ells from rat aorta. The SMA mRNA is also present at all stages of dev elopment in the smooth muscles examined; however, it is much less abun dant than SMB mRNA in most fetal smooth muscles. We show here that the SMB isoform, which contains a unique seven-amino acid insertion at th e junction of the 25- and 50-kDa tryptic peptides, is present in conju nction with SM1 and SNIB tails on immunoblots of smooth muscle from st omach, intestine, bladder, and uterus and is expressed during developm ent in a pattern distinct from that of the SM1 and SM2 tail isoforms.