TEMPORAL AND SPATIAL EXPRESSION OF DISTINCT TROPONIN-T GENES IN EMBRYONIC LARVAL TAIL STRIATED-MUSCLE AND ADULT BODY-WALL SMOOTH-MUSCLE OF ASCIDIAN/

During development of the ascidian Halocynthia roretzi, the tadpole la rva hatched from the tailbud embryo metamorphoses to the adult with a body wall muscle. Although the adult body wall muscle is morphological ly nonsarcomeric smooth muscle, it contains a troponin complex consist ing of three subunits (T, I, and C) as do vertebrate striated muscles. Different from vertebrate troponins, however, the smooth muscle tropo nin promotes actin-myosin interaction in the presence of high concentr ation of Ca2+, and this promoting property Is attributable to troponin T. To address whether the embryonic/larval tail striated muscle and t he adult smooth muscle utilize identical or different regulatory machi nery, we cloned troponin T cDNAs from each cDNA library, The embryonic and the adult troponin Ts were encoded by distinct genes and shared o nly <60% identity with each other. These isoforms were specifically ex pressed in the embryonic/larval tail striated muscle and the adult smo oth muscle, respectively. These results may imply that these isoforms regulate actin-myosin interaction in different manners. The adult trop onin T under forced expression in mouse fibroblasts was unexpectedly l ocated in the nuclei. However, a truncated protein with a deletion inc luding a cluster of basic amino acids colocalized with tropomyosin on actin filaments. Thus, complex formation with troponin I and C immedia tely after the synthesis is likely to be essential for the protein to properly localize on the thin filaments.