Abstract：
In the classic molecular model of nacreous layer formation, unusual acidic matrix proteins rich in aspartic acid (Asp) residues are essential for nacre nucleation due to their great affinity for binding calcium. However, the acidic matrix proteins discovered in the nacreous layer so far have been weakly acidic with a high proportion of glutamate. In the present study, several silk-like matrix proteins, including the novel matrix protein HcN57, were identified in the ethylenediaminetetraacetic acid-soluble extracts of the nacreous layer of Hyriopsis cumingii. HcN57 is a highly repetitive protein that consists of a high proportion of alanine (Ala, 34.4%), glycine (Gly, 22.5%), and serine (Ser, 11.4%). It forms poly Ala blocks, GlynX repeats, an Ala-Gly repeat, and a Ser-Ala-rich region, exhibiting significant similarity to silk proteins found in spider species. The expression of HcN57 was specifically located in the dorsal epithelial cells of the mantle pallium and mantle center. Notably, expression of HcN57 was relatively high during nacreous layer regeneration and pearl nacre deposition, suggesting HcN57 is a silk matrix protein in the nacreous layer. Importantly, HcN57 also contains a certain content of Asp residues, making it an unusual acidic matrix protein present in the nacreous layer. These Asp residues are mainly distributed in three large hydrophilic acidic regions, which showed inhibitory activity against aragonite deposition and morphological regulation of calcite in vitro. Moreover, HcN57-dsRNA injection resulted in failure of nacre nucleation in vivo. Taken together, our results show that HcN57 is a bifunctional silk protein with poly Ala blocks and Gly-rich regions that serve as space fillers within the chitinous framework to prevent crystallization at unnecessary nucleation sites and Asp-rich regions that create a calcium ion supersaturated microenvironment for nucleation in the center of nacre tablets. These observations contribute to a better understanding of the mechanism by which silk proteins regulate framework construction and nacre nucleation during nacreous layer formation.
摘要：
在珠光层形成的经典分子模型中，富含天冬氨酸（Asp）残基的不寻常的酸性基质蛋白对于珍珠质成核至关重要，因为它们对结合钙的亲和力很高。然而,到目前为止，在珠光层中发现的酸性基质蛋白是弱酸性的，谷氨酸含量很高。在本研究中,几种丝样基质蛋白，包括新的基质蛋白HcN57，已在蓬松沙棘珠光层的乙二胺四乙酸可溶性提取物中鉴定。HcN57是一种高度重复的蛋白质，由高比例的丙氨酸组成(Ala，34.4%），甘氨酸(Gly,22.5%），和丝氨酸(Ser，11.4%）。它形成了多边形Ala块，GlynX重复,Ala-Gly重复,和一个富含Ser-Ala的地区，与蜘蛛物种中发现的丝蛋白表现出明显的相似性。HcN57的表达特异性地定位在地幔皮层和地幔中心的背侧上皮细胞中。值得注意的是,HcN57的表达在珠光层再生和珍珠质沉积过程中相对较高，表明HcN57是珠光层中的丝基质蛋白。重要的是,HcN57还含有一定含量的Asp残基，使其成为珠光层中存在的不寻常的酸性基质蛋白。这些Asp残基主要分布在三个大的亲水性酸性区域,具有体外抑制方解石沉积和形态调控的活性。此外，HcN57-dsRNA注射导致珍珠母体内成核失败。一起来看,我们的结果表明，HcN57是一种双功能丝蛋白，具有聚Ala嵌段和富含Gly的区域，可作为几丁质框架内的空间填料，以防止在不必要的成核位点和富含Asp的区域结晶，从而产生钙离子过饱和的微环境在珍珠层片剂的中心成核。这些观察结果有助于更好地理解丝蛋白在珍珠层形成过程中调节骨架结构和珍珠层成核的机制。
