HcTyr and HcTyp-1 of Hyriopsis cumingii, novel tyrosinase and tyrosinase-related protein genes involved in nacre color formation

Tyrosinase is an important enzyme that is involved in biological processes such as pigmentation, wound healing, sclerotization of the cuticles, oxygen transport and innate immunity. As nacre color has an effect on pearl color, we studied the effect of tyrosinase on nacre color in Hyriopsis cumingii (an important freshwater pearl-producing mussel) by cloning novel tyrosinase protein and tyrosinase-related protein genes (HcTyr and HcTyp-1 respectively) from the mantle. The predicted amino acid sequences of HcTyr and HcTyp-1 contain conserved domains, and HcTyp-1 contains an additional chitin-binding domain. Two different types of mussels, purple shelled and white shelled, were used to investigate the role of tyrosinase in shell color. HcTyr and HcTyp-1 mRNAs were mainly expressed in the mantle, but the expression of HcTyr was higher in the purple mussel than in the white mussel while the expression of HcTyp-1 was higher in the white mussel. Strong and specific mRNA signals for HcTyp-1 were detected in the dorsal epithelial cells of the mantle pallial and some signals were detected in the epithelial cells of the periostracal groove, so HcTyp-1 may be involved in periostracum and nacreous layer formation. Strong and specific mRNA signals were also detected in the dorsal epithelial cells of the mantle pallial, so HcTyr may be involved in nacre formation. Further, the tyrosinase activity of the mantle in the purple mussel was higher than that in the white mussel. These findings indicate that HcTyr and HcTyp-1 may be involved in the formation of nacre color in H. cumingii.