Striated muscle-type tropomyosin in a chordate smooth muscle, ascidian body-wall muscle.
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.
Meedel
T H
TH
Department of Biology, Rhode Island College, Providence 02908.
Hastings
K E
KE
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D015152:Blotting, Northern; D015139:Blotting, Southern; D002938:Ciona intestinalis / Q000737:chemistry* / Q000235:genetics; D004247:DNA; D008969:Molecular Sequence Data; D009130:Muscle, Smooth / Q000737:chemistry*; D009206:Myocardium / Q000378:metabolism; D016133:Polymerase Chain Reaction; D051381:Rats; D017386:Sequence Homology, Amino Acid; D014335:Tropomyosin / Q000032:analysis* / Q000235:genetics
D014335:Tropomyosin; D004247:DNA