Calponin Isoform Expression in the Japanese Pearl Oyster, Pinctada fucata

Calponin is a basic actin-binding protein found widely in invertebrate tissues including catch muscle and therefore may participate in catch contraction. There is limited information about molluscan calponin and molecular characterization to reveal its function in the regulatory system. We previously identified and partially sequenced three calponin isoforms of the Japanese pearl oyster, Pinctada fucata (Pifuc-CP-1, Pifuc-CP-2 and Pifuc-CP-3). In this study, the full-length nucleotide sequences of the three isoforms were determined. The primary structures revealed that Pifuc-CP-1 consists of 324 amino acids (aa) with a molecular mass (Mw) of 34.7 kDa and an isoelectric point (pI) of 9.40. Pifuc-CP-2 is 303 aa in length with a Mw of 33.3 kDa and a pI of 9.30, and Pifuc-CP-3 is 398 aa in length with a Mw of 43.8 kDa and a pI of 8.55. Domain architecture prediction showed that the three isoforms have a single calponin homology (CH) domain and multiple calponin (CN) domains. Pifuc-CP-1, Pifuc-CP-2 and Pifuc-CP-3 possess four, three and five CN domains, respectively. Tissue distribution analysis indicated the presence of additional calponin isoforms and these isoforms are distributed widely in muscle and non-muscle tissues. Results of cDNA cloning revealed further four calponin isoforms: Pifuc-CP-4 (402 aa, 42.8 kDa, pI = 9.10), Pifuc-CP-5 (285 aa, 30.7 kDa, pI = 9.45), Pifuc-CP-6 (286 aa, 31.1 kDa, pI = 9.60) and Pifuc-CP-7 (302 aa, 33.3 kDa, pI = 9.10). The domain architecture of these four isoforms also consists of a single CH domain and multiple CN domains. Pifuc-CP-4 possesses six CN domains, whereas Pifuc-CP-5, Pifuc-CP-6 and Pifuc-CP-7 contain three CN domains. Sequence alignment of P. fucata calponin isoforms showed that Pifuc-CP-1, Pifuc-CP-2, Pifuc-CP-3 and Pifuc-CP-4 have identical CH domain sequences, whereas Pifuc-CP-5, Pifuc-CP-6 and Pifuc-CP-7 have identical CH domain sequences. The CN repeats were not well conserved. These findings suggest that P. fucata calponin isoforms function differently in each tissue.