DISTINCT TROPONIN-T GENES ARE EXPRESSED 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 sessile adult with a body wall muscle. Although the adult body wall muscle is morph ologically nonsarcomeric smooth muscle, it contains troponin complex c onsisting of three subunits (T, I, and C) as do vertebrate striated mu scles. Different from vertebrate troponins, however, the smooth muscle troponin promotes actomyosin Mg2+-ATPase activity in the presence of high concentration of Ca2+, and this promoting property is attributabl e to troponin T. To address whether the embryonic/larval tail striated muscle and the adult smooth muscle utilize identical or different reg ulatory machinery, we cloned troponin T cDNAs from each cDNA library. The embryonic and the adult troponin Ts were encoded by distinct genes and shared only <60% identity with each other. Northern blotting and whole mount in situ hybridization revealed that these isoforms were sp ecifically expressed in the embryonic/larval tail striated muscle and the adult smooth muscle, respectively. These results may imply that th ese isoforms regulate actin-myosin interaction in different manners. T he adult troponin T under forced expression in mouse fibroblasts was u nexpectedly located in the nuclei. However, a truncated protein with a deletion including a cluster of basic amino acids colocalized with tr opomyosin on actin filaments. Thus, complex formation with troponin I and C immediately after the synthesis is likely to be essential for th e protein to properly localize on the thin filaments.