Distinct Troponin T Genes Are Expressed in Embryonic/Larval Tail Striated Muscle and Adult Body Wall Smooth Muscle of Ascidian

doi:10.1074/jbc.271.44.27855

Distinct Troponin T Genes Are Expressed in Embryonic/Larval Tail Striated Muscle and Adult Body Wall Smooth Muscle of Ascidian*

From the Department of Biology, Faculty of Science, Chiba University, Yayoicho, Inageku, Chiba 263, Japan

^‡To whom correspondence should be addressed. Tel./Fax: 81-43-290-3911; E-mail: tendo{at}cuphd.nd.chiba-u.ac.jp.

Abstract

During development of the ascidian Halocynthia roretzi, the tadpole larva hatched from the tailbud embryo metamorphoses to the sessile adult with a body wall muscle. Although the adult body wall muscle is morphologically nonsarcomeric smooth muscle, it contains troponin complex consisting of three subunits (T, I, and C) as do vertebrate striated muscles. 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 attributable to troponin T. To address whether the embryonic/larval tail striated muscle and the adult smooth muscle utilize identical or different regulatory 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 specifically expressed in the embryonic/larval tail striated muscle and the adult smooth muscle, respectively. These results may imply that these isoforms regulate actin-myosin interaction in different manners. The adult troponin T under forced expression in mouse fibroblasts was unexpectedly located in the nuclei. However, a truncated protein with a deletion including a cluster of basic amino acids colocalized with tropomyosin on actin filaments. Thus, complex formation with troponin I and C immediately after the synthesis is likely to be essential for the protein to properly localize on the thin filaments.

Footnotes

↵^§ Recipient of Research Fellowship of Japan Society for the Promotion of Science.
↵* This study was supported by grants-in-aid for scientific research from the Ministry of Education, Science, and Culture of Japan. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank™/EMBL Data Bank with accession number(s) D50867[GenBank] and D85077[GenBank].
↵¹ The abbreviations used are:

Tn

troponin

TnT

troponin T

TnI

troponin I

TnC

troponin C

Tm

tropomyosin

PAGE

polyacrylamide gel electrophoresis

bp

base pair(s)

adTnT

H. roretzi TnT encoded by the cDNAs aTnT2 and aTnT19

embTnT

TnT encoded by eTnT11

NLS

nuclear localization signal.
↵²N. Sawada and T. Endo, unpublished observations.
- Received May 8, 1996.
- Revision received July 26, 1996.