Hic74, a novel alanine and glycine rich matrix protein related to nacreous layer formation in the mollusc Hyriopsis cumingii

Organic matrix proteins play an important role in the biomineralization of mollusc shells. We have identified and characterized a novel matrix protein, namely hic74, isolated from the mantle of the freshwater mussel Hyriopsis cumingii. The hic74 gene encodes a 850 amino acid protein that is rich in alanine (ala, 30.8%), glycine (gly, 25.8%) and serine (ser, 10.6%), with ala mainly existing in poly-ala forms within the ala/gly-rich regions. Quantitative PCR expression analysis demonstrated that hic74 was specifically amplified from the mantle and in situ hybridization showed a strong signal in the epithelial cells in the pallial region of the mantle. Lower levels of expression were detected during the early stages of pearl sac formation but increased and remained constant during nacreous layer formation suggesting that hic74 might be involved in this process.