TITLE:
Molecular Cloning and Tissue Distribution of Troponin I from the Japanese Pearl Oyster, Pinctada fucata
AUTHORS:
Daisuke Funabara, Yoshinori Urakawa, Satoshi Kanoh
KEYWORDS:
Adductor Muscle, Catch Contraction, Pinctada fucata, Troponin, Troponin I
JOURNAL NAME:
American Journal of Molecular Biology,
Vol.9 No.2,
March
5,
2019
ABSTRACT: Troponin is a complex of three proteins (troponin I,
troponin C, and troponin T) that binds Ca2+ and is a thin filament-associated
regulator of vertebrate striated muscle contraction. The function of troponin I
(TnI) in vertebrates has been extensively
characterized, but its role in molluscan muscles has not yet been elucidated.
Our previous work suggested that the troponin C subunit has a role in adductor
phasic muscle but not in catch muscle. Here, we investigated the molecular
characteristics of TnI from the bivalve Japanese pearl oyster, Pinctada fucata to aid the elucidation
of the function of molluscan muscle troponin. We determined the primary
structure of the full-length TnI protein from the P. fucata adductor muscle (Pifuc-TnI) and found that it is composed
of 286 amino acid residues with a predicted molecular weight of 33,737. Motif
structure predictions and multiple sequence alignments revealed that Pifuc-TnI
has a 138 residue extension at its N-terminus compared with rabbit TnI. This is analogous to characterized TnIs from other mollusks. However,
unlike scallop TnI, Pifuc-TnI is predicted to contain two cAMP-dependent
protein kinase phosphorylation sites, at residues 39 - 45 (RRGTEDD) and 145 - 151 (KKKSKRK).
Phylogenetic analysis indicated that Pifuc-TnI and molluscan TnIs were grouped
into the same clade. Pifuc-TnI gene structure predictions using
Splign alignment of our obtained cDNA and genome sequences indicated that Pifuc-TnI consists of fifteen exons, with the start and stop codons
located in exon 2 and exon 11, respectively. Using quantitative real-time PCR,
we determined that the Pifuc-TnI gene is predominantly expressed in
adductor phasic muscle, weakly in adductor catch muscle, and is not expressed
in the gill, mantle or foot. These findings suggest that TnI, as a component of
the troponin complex, plays a regulatory role in adductor phasic muscle
contraction, but not in catch contraction.