Striated muscle-type tropomyosin in a chordate smooth muscle, ascidian body-wall muscle

• Published in: The Journal of biological chemistry 1993-03, Vol.268 (9), p.6755-6764
• Main Authors: MEEDEL, T. H, HASTINGS, K. E. M
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Analytical, structural and metabolic biochemistry; Animals; Base Sequence; Biological and medical sciences; Blotting, Northern; Blotting, Southern; body walls; Ciona intestinalis; Ciona intestinalis - chemistry; Ciona intestinalis - genetics; Contractile proteins; DNA; expression; Fundamental and applied biological sciences. Psychology; Holoproteins; isoforms; Marine; Molecular Sequence Data; Muscle, Smooth - chemistry; muscles; Myocardium - metabolism; Polymerase Chain Reaction; Proteins; Rats; Sequence Homology, Amino Acid; smooth muscle; tropomyosin; Tropomyosin - analysis; Tropomyosin - genetics
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/s0021-9258(18)53314-7

Body-wall muscle tropomyosin (Tm) of a marine chordate, the ascidian Ciona intestinalis, was studied by protein and cDNA clone analyses. Our results indicate that body-wall muscle of Ciona contains one major Tm isoform encoded by a single gene. Unexpectedly, the sequence of this Tm resembles vertebrate-striated muscle Tm isoforms, rather than those of smooth muscle or nonmuscle tissues, despite the fact that body-wall muscle is a nonsarcomeric (i.e. smooth) muscle. We also found that an apparently identical Tm isoform, derived from the same gene, is expressed at high levels in Ciona heart, a striated muscle. This is the first example of an organism in which a single Tm isoform is prominently expressed in both sarcomeric and non-sarcomeric tissues. Our results demonstrate that the characteristic features of "sarcomeric" Tm isoforms are not primarily related to sarcomeric ultrastructure per se. Instead, because ascidian body-wall muscle, unlike vertebrate smooth muscle, contains troponin, we suggest that it is the interaction with troponin that generates the selective pressure to maintain the characteristic C-terminal structure of so-called sarcomeric Tm isoforms. Our results further document the remarkable molecular similarity between the nonsarcomeric ascidian body-wall muscle and vertebrate-striated muscle. We suggest that these muscle types represent sarcomeric and nonsarcomeric variants of a fundamental class of troponin/Tm-regulated muscles, contrary to the traditional smooth/striated classification of muscle types. The possible relationship of this class of muscle to vertebrate smooth muscle is discussed.