School of Life and Environmental Sciences, Shaoxing University, Shaoxing, China.
Key Laboratory of Freshwater Aquatic Genetic Resources, Ministry of Agriculture and Rural Affairs, Shanghai Ocean University, Shanghai, 201306, China.
Mar Biotechnol (NY). 2024 Aug;26(4):716-731. doi: 10.1007/s10126-024-10339-3. Epub 2024 Jun 19.
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, GlyX 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.
在经典的珍珠层形成的分子模型中，富含天冬氨酸（Asp）残基的特殊酸性基质蛋白对于珍珠核的形成至关重要，因为它们与钙结合的亲和力很强。然而，到目前为止，在珍珠层中发现的酸性基质蛋白一直是弱酸性的，谷氨酸的比例很高。在本研究中，从 Hyriopsis cumingii 的珍珠层的乙二胺四乙酸可溶提取物中鉴定出几种丝氨酸样基质蛋白，包括新型基质蛋白 HcN57。HcN57 是一种高度重复的蛋白，由高比例的丙氨酸（Ala，34.4%）、甘氨酸（Gly，22.5%）和丝氨酸（Ser，11.4%）组成。它形成聚 Ala 块、GlyX 重复、Ala-Gly 重复和富含 Ser-Ala 的区域，与蜘蛛物种中发现的丝蛋白具有显著的相似性。HcN57 的表达特异性位于外套膜的背上皮细胞和外套膜中心。值得注意的是，在珍珠层再生和珍珠质沉积过程中，HcN57 的表达相对较高，这表明 HcN57 是珍珠层中的丝基质蛋白。重要的是，HcN57 还含有一定含量的 Asp 残基，使其成为珍珠层中存在的特殊酸性基质蛋白。这些 Asp 残基主要分布在三个大的亲水酸性区域，这些区域在体外对文石沉积具有抑制活性，并对方解石的形态调节具有抑制活性。此外，HcN57-dsRNA 注射导致体内珍珠核形成失败。综上所述，我们的结果表明，HcN57 是一种具有聚 Ala 块和富含 Gly 区域的双功能丝蛋白，作为壳质框架内的空间填充物，防止在不必要的成核部位结晶，并富含 Asp 区域，为珍珠层薄片中心的成核创造钙离子过饱和的微环境。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框架构建和珍珠核形成的机制。
