Structure of 29-kDa protein from ascidian (Halocynthia roretzi) body wall muscle

Preparations of thin filament from ascidian (Halocynthia roretzi) body wall muscle were found to be contaminated with large amounts of an unknown protein which, when purified, migrated to 29 kDa on SDS-PAGE and was thus named HR-29. To elucidate its physiological function, we have determined the primary structure of HR-29 by peptide and cDNA sequence analysis. Genomic sequence analysis showed that HR-29 is divided to four exons by three introns. It is composed of 251 amino acid residues and the N-terminal Ser is blocked by an acetyl group. The N-terminal domain of 150 residues is hydrophilic, and from residue 36 to 92 there are three similar repeated sequences of 19 residues. The N-terminal domain showed no significant sequence homology with other proteins. On the other hand, the C-terminal domain of 100 residues has homology with those of small heat-shock proteins and alpha-crystallin of lens protein. HR-29 is thought to be a fusion protein of two different origins. Recently alpha B-crystallin was identified in skeletal muscle and suggested to be a myofibril-stabilizing protein [Atomi, T. et al. (1991) J. Biochem. 110, 2360-2364]. Thus, HR-29 may also be a myofibril-stabilizing protein, as suggested by the sequence similarity of the C-terminal domain with alpha-crystallin, although the origin and role of the N-terminal domain are not clear.