Dual Roles of the Lysine-Rich Matrix Protein (KRMP)-3 in Shell Formation of Pearl Oyster, <i>Pinctada fucata</i>

<div><p>Matrix proteins play important roles in shell formation. Our group firstly isolated three cDNAs encoding lysine-rich matrix protein from <i>Pinctada fucata</i> in 2006. However, the functions of KRMPs are not fully understood. In addition, KRMPs contain two functional domains, the basic domain and the Gly/Tyr domain respectively. Based on the modular organization, the roles of their two domains were poorly characterized. Furthermore, KRMPs were then reported in other two species, <i>P</i>. <i>maxima</i> and <i>P</i>. <i>margaritifera</i>, which indicated that KRMPs might be very important for shell formation. In this study, the characterization and function of KRMP-3 and its two functional domains were studied <i>in vitro</i> through purification of recombinant glutathione S-transferase tagged KRMP-3 and two KRMP-3 deletion mutants. Western blot and immunofluorescence revealed that native KRMP-3 existed in the EDTA-insoluble matrix of the prismatic layer and was located in the organic sheet and the prismatic sheath. Recombinant KRMP-3 (rKRMP-3) bound tightly to chitin and this binding capacity was duo to the Gly/Tyr-rich region. rKRMP-3 inhibited the precipitation of CaCO₃, affected the crystal morphology of calcite and inhibited the growth of aragonite <i>in vitro</i>, which was almost entirely attributed to the lysine-rich region. The results present direct evidence of the roles of KRMP-3 in shell biomineralization. The functional rBR region was found to participate in the growth control of crystals and the rGYR region was responsible to bind to chitin.</p>