BACKGROUND The Drosophila melanogaster Serpin 42 Da gene (previously Serpin 4) encodes a serine protease inhibitor that is capable of remarkable functional diversity through the alternative splicing of four different reactive centre loop exons. Eight protein isoforms of Serpin 42 Da have been identified to date, targeting the protease inhibitor to both different proteases and cellular locations. Biochemical and genetic studies suggest that Serpin 42 Da inhibits target proteases through the classical serpin 'suicide' inhibition mechanism, however the crystal structure of a representative Serpin 42 Da isoform remains to be determined. RESULTS We report two high-resolution crystal structures of Serpin 42 Da representing the A/B isoforms in the cleaved conformation, belonging to two different space-groups and diffracting to 1.7 Å and 1.8 Å. Structural analysis reveals the archetypal serpin fold, with the major elements of secondary structure displaying significant homology to the vertebrate serpin, neuroserpin. Key residues known to have central roles in the serpin inhibitory mechanism are conserved in both the hinge and shutter regions of Serpin 42 Da. Furthermore, these structures identify important conserved interactions that appear to be of crucial importance in allowing the Serpin 42 Da fold to act as a versatile template for multiple reactive centre loops that have different sequences and protease specificities. CONCLUSIONS In combination with previous biochemical and genetic studies, these structures confirm for the first time that the Serpin 42 Da isoforms are typical inhibitory serpin family members with the conserved serpin fold and inhibitory mechanism. Additionally, these data reveal the remarkable structural plasticity of serpins, whereby the basic fold is harnessed as a template for inhibition of a large spectrum of proteases by reactive centre loop exon 'switching'. This is the first structure of a Drosophila serpin reported to date, and will provide a platform for future mutational studies in Drosophila to ascertain the functional role of each of the Serpin 42 Da isoforms.

中文翻译：

---

黑腹果蝇裂解的 Serpin 42 Da 的高分辨率结构。

背景 黑腹果蝇 Serpin 42 Da 基因（以前称为 Serpin 4）编码一种丝氨酸蛋白酶抑制剂，该抑制剂能够通过四个不同反应中心环外显子的选择性剪接实现显着的功能多样性。迄今为止，已鉴定了 Serpin 42 Da 的八种蛋白质亚型，将蛋白酶抑制剂靶向不同的蛋白酶和细胞位置。生化和遗传研究表明，Serpin 42 Da 通过经典的 Serpin“自杀”抑制机制抑制目标蛋白酶，但代表性 Serpin 42 Da 同种型的晶体结构仍有待确定。结果我们报告了两种高分辨率的 Serpin 42 Da 晶体结构，代表裂解构象中的 A/B 异构体，属于两个不同的空间群，衍射到 1.7 Å 和 1.8 Å。结构分析揭示了原型丝氨酸蛋白酶抑制剂折叠，二级结构的主要元素与脊椎动物丝氨酸蛋白酶抑制剂神经丝氨酸蛋白酶抑制剂具有显着的同源性。已知在丝氨酸蛋白酶抑制剂抑制机制中具有核心作用的关键残基在丝氨酸蛋白酶抑制剂 42 Da 的铰链和快门区域中都是保守的。此外，这些结构确定了重要的保守相互作用，这些相互作用似乎对于允许 Serpin 42 Da 折叠作为具有不同序列和蛋白酶特异性的多个反应中心环的通用模板发挥至关重要的作用。结论 结合先前的生化和遗传研究，这些结构首次证实 Serpin 42 Da 亚型是典型的抑制性 serpin 家族成员，具有保守的 serpin 折叠和抑制机制。此外，这些数据揭示了丝氨酸蛋白酶抑制剂显着的结构可塑性，其中基本折叠被用作模板，通过反应性中心环外显子“转换”抑制大范围的蛋白酶。这是迄今为止报道的第一个果蝇 serpin 结构，将为未来果蝇突变研究提供一个平台，以确定每个 Serpin 42 Da 亚型的功能作用。