Relationship between alternatively spliced exons and functional domains in tropomyosin.
Smooth and striated muscle alpha-tropomyosins differ as a consequence of alternative splicing of exons 2 and 9 encoding amino acid residues 39-80 and 258-284, respectively [Ruiz-Opazo, N., Weinberger, J. & Nadal-Ginard, B. (1985) Nature (London) 315, 67-70]. To understand the relationship between alternatively spliced exons and functional domains in tropomyosin, recombinant unacetylated striated muscle, smooth muscle, and chimeric rat alpha-tropomyosins (+H3N-tropomyosins) expressed in and purified from Escherichia coli were analyzed. The functional differences between the isoforms can be primarily ascribed to exon 9. +H3N-Tropomyosins with the smooth muscle exon 9 bound to skeletal muscle filamentous actin with at least a 5-fold higher affinity than +H3N-tropomyosins with the striated muscle exon 9. On the other hand, in the presence of Ca2+, troponin increased the affinity of +H3N-tropomyosins with the striated muscle exon 9 at least 50-fold, whereas it had little effect on +H3N-tropomyosins with the smooth muscle exon 9. The unique striated muscle alpha-tropomyosin exon 9 seems to be specialized for Ca(2+)-insensitive interaction with troponin on the thin filament. The unique smooth muscle alpha-tropomyosin exon 2 was associated with a slightly lower actin affinity than the striated muscle exon 2. Although the regions encoded by exons 2 and 9 correspond to functional domains, they are not recognizable as independent units or structural domains in the extended coiled-coil structure of this fibrous actin binding protein.
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Joint Graduate Program in Biochemistry and Molecular Biology, Rutgers University, Piscataway, NJ 08854.
Hitchcock-DeGregori
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D000199:Actins / Q000378:metabolism; D000818:Animals; D002678:Chimera; D003001:Cloning, Molecular; D004247:DNA / Q000235:genetics; D005091:Exons; D005260:Female; D007700:Kinetics; D009130:Muscle, Smooth / Q000502:physiology; D009132:Muscles / Q000502:physiology; D009218:Myosins / Q000378:metabolism; D011485:Protein Binding; D012326:RNA Splicing; D051381:Rats; D011994:Recombinant Proteins / Q000302:isolation & purification / Q000378:metabolism; D013270:Stomach / Q000502:physiology; D014335:Tropomyosin / Q000235:genetics* / Q000302:isolation & purification / Q000378:metabolism; D014599:Uterus / Q000502:physiology
D000199:Actins; D011994:Recombinant Proteins; D014335:Tropomyosin; D004247:DNA; D009218:Myosins