title: Three novel brain tropomyosin isoforms are expressed from the rat alpha-tropomyosin gene through the use of alternative promoters and alternative RNA processing.
abstract: cDNA clones encoding three novel tropomyosins, termed TMBr-1, TMBr-2, and TMBr-3, were isolated and characterized from a rat brain cDNA library. All are derived from a single gene, which was previously found to express striated muscle alpha-tropomyosin and a number of other tropomyosin isoforms via an alternative splicing mechanism (N. Ruiz-Opazo and B. Nadal-Ginard, J. Biol. Chem. 262:4755-4765, 1987; D. F. Wieczorek, C. W. J. Smith, and B. Nadal-Ginard, Mol. Cell. Biol. 8:679-694, 1988). The derived amino acid sequences revealed that TMBr-1 contains 281 amino acids, TMBr-2 contains 251 amino acids, and TMBr-3 contains 245 amino acids. All three proteins contain a region that is identical to amino acids 81 through 258 of skeletal muscle alpha-tropomyosin. TMBr-1 is identical to striated muscle alpha-tropomyosin from amino acids 1 through 258 but contains a novel COOH-terminal region from amino acids 259 through 281. TMBr-2 and TMBr-3 both contain identical NH2-terminal sequences from amino acids 1 through 44 which were found to be expressed from a novel promoter. TMBr-3 contains the same COOH-terminal region as TMBr-1, whereas TMBr-2 contains a second novel COOH-terminal region. The genomic organization of the exons encoding TMBr-1, TMBr-2, and TMBr-3 were determined. These studies revealed a previously uncharacterized promoter located in the internal region of the alpha-TM gene as well as two novel COOH-terminal coding exons. The alpha-TM gene is a complex transcription unit containing 15 exons including two alternative promoters, two internal mutually exclusive exon cassettes, and four alternatively spliced 3' exons that encode four different COOH-terminal coding regions. A total of nine distinct mRNAs are known to be expressed from the alpha-TM gene in a cell type-specific manner in tissues such as striated muscle, smooth muscle, kidney, liver, brain, and fibroblasts. The mRNAs encoding TMBr-1, TMBr-2, and TMBr-3 were found to be expressed only in brain tissue, with TMBr-3 being expressed at much greater levels than TMBr-1 and TMBr-2. The individual structural characteristics of each brain alpha-tropomyosin isoform and their possible functions are discussed.
authors.affiliation: Cold Spring Harbor Laboratory, New York 11724.
authors.forename: J P
authors.initials: JP
authors.lastname: Lees-Miller
authors.forename: L O
authors.initials: LO
authors.lastname: Goodwin
authors.forename: D M
authors.initials: DM
authors.lastname: Helfman
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001921:Brain / Q000378:metabolism*; D003001:Cloning, Molecular; D005091:Exons; D015723:Gene Library; D005796:Genes; D014644:Genetic Variation; D007438:Introns; D008969:Molecular Sequence Data; D010957:Plasmids; D011401:Promoter Regions, Genetic; D012326:RNA Splicing; D012333:RNA, Messenger / Q000235:genetics; D051381:Rats; D015183:Restriction Mapping; D012689:Sequence Homology, Nucleic Acid; D014158:Transcription, Genetic; D014335:Tropomyosin / Q000235:genetics*
chemical_list: D012333:RNA, Messenger; D014335:Tropomyosin