Nacre formation is an ideal model to study biomineralization processes. Although much has been done about biomineralization mechanism of nacre, little is known as to how cellular signaling regulates this process. We are interested in whether G protein signaling plays a role in mineralization. Degenerate primers against conserved amino acid regions of G proteins were employed to amplify cDNA from the pearl oyster Pinctada fucata. As a result, the cDNA encoding a novel Gsa (pfGsa) from the pearl oyster was isolated. The Gsa cDNA encodes a polypeptide of 377 amino acid residues, which shares high similarity to the octopus (Octopus vulgaris) Gsa. The well-conserved A, C, G (switch I), switch II functional domains and the carboxyl terminus that is a critical site for interaction with receptors are completely identical to those from other mollusks. However, pfGsa has a unique amino acid sequence, which encodes switch III and interaction sites of adenylyl cyclase respectively. In situ hybridization and Northern blotting analysis revealed that the oyster Gsa mRNA is widely expressed in a variety of tissues, with highest levels in the outer fold of mantle and epithelia of gill, the regions essential for biomineralization. We also show that overexpression of the pfGsa in mammalian MC3T3-E1 cells resulted in increased cAMP levels. Mutant pfGsa that has impaired CTX substrate diminished its ability to induce cAMP production. Furthermore, the alkaline phosphatase (ALP) activity, an indicator for mineralization, is induced by the Gsa in MC3T3-E1 cells. These results indicated that Gsa may be involved in regulation of physiological function, particularly in biological biomineralization.