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.