The N-terminal polypeptide of a new shell matrix protein hicraqin accelerates the rate of calcium carbonate deposition

Matrix proteins play important roles in shell formation by regulating the assembly of organic matrix and minerals. Here, we obtained a new matrix protein (hicraqin) from Hyriopsis cumingii. The amino acid sequence of hicraqin contains multiple aggregated (Gly)n (n > 2) residues, a feature unique to silk-like matrix proteins. In situ hybridization studies and tissue expression patterns demonstrated that hicraqin may be a prismatic layer matrix protein. In vitro experiments were performed using the peptide N-hicraqin (the N-terminal free sequence of hicraqin). In the in vitro crystallization of calcium carbonate, crystals resembling dumbbell, spindle, and lotus aragonite crystals were observed under scanning electron microscopy and confirmed as calcite by Raman spectroscopy. In the in vitro crystallization system of calcium carbonate with the addition of magnesium ions, aragonite plates were generated with 50 μg/mL of the peptide N-hicraqin. The fluorescent labeling analysis indicated that N-hicraqin was involved in the crystallization process. The crystallization rate experiment showed that the peptide N-hicraqin plays a role in promoting crystallization. Following the silencing of the hicraqin gene by RNA interference, its expression was reduced by about 61 %. There was incomplete formation of the organic framework outside the prismatic layer. Overall, the present study showed that N-hicraqin participates in the crystallization process and acts as a framework protein that influences the formation of the organic framework of the prismatic layer.