Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461, USA.
Department of Biochemistry and Biophysics, University of California San Francisco, San Francisco, CA 94158-2517, USA.
J Mol Biol. 2014 Apr 3;426(7):1452-68. doi: 10.1016/j.jmb.2013.12.009. Epub 2013 Dec 12.
Fusion of the viral and host cell membranes is a necessary first step for infection by enveloped viruses and is mediated by the envelope glycoprotein. The transmembrane subunits from the structurally defined "class I" glycoproteins adopt an α-helical "trimer-of-hairpins" conformation during the fusion pathway. Here, we present our studies on the envelope glycoprotein transmembrane subunit, GP2, of the CAS virus (CASV). CASV was recently identified from annulated tree boas (Corallus annulatus) with inclusion body disease and is implicated in the disease etiology. We have generated and characterized two protein constructs consisting of the predicted CASV GP2 core domain. The crystal structure of the CASV GP2 post-fusion conformation indicates a trimeric α-helical bundle that is highly similar to those of Ebola virus and Marburg virus GP2 despite CASV genome homology to arenaviruses. Denaturation studies demonstrate that the stability of CASV GP2 is pH dependent with higher stability at lower pH; we propose that this behavior is due to a network of interactions among acidic residues that would destabilize the α-helical bundle under conditions where the side chains are deprotonated. The pH-dependent stability of the post-fusion structure has been observed in Ebola virus and Marburg virus GP2, as well as other viruses that enter via the endosome. Infection experiments with CASV and the related Golden Gate virus support a mechanism of entry that requires endosomal acidification. Our results suggest that, despite being primarily arenavirus like, the transmembrane subunit of CASV is extremely similar to the filoviruses.
病毒和宿主细胞膜的融合是包膜病毒感染的必要的第一步，由包膜糖蛋白介导。结构定义的“I 类”糖蛋白的跨膜亚基在融合途径中采用α-螺旋“发夹三聚体”构象。在这里，我们介绍了我们对 CAS 病毒（CASV）包膜糖蛋白跨膜亚基 GP2 的研究。CASV 最近从患有包涵体病的环纹树蟒（Corallus annulatus）中被鉴定出来，并且与疾病的病因有关。我们已经生成并表征了由预测的 CASV GP2 核心结构域组成的两种蛋白质构建体。CASV GP2 融合后构象的晶体结构表明，形成了三聚体α-螺旋束，尽管 CASV 基因组与沙粒病毒科病毒具有同源性，但与埃博拉病毒和马尔堡病毒 GP2 非常相似。变性研究表明，CASV GP2 的稳定性取决于 pH 值，在较低 pH 值下稳定性更高；我们提出这种行为是由于酸性残基之间的相互作用网络所致，在侧链去质子化的情况下，会破坏α-螺旋束的稳定性。埃博拉病毒和马尔堡病毒 GP2 以及其他通过内体进入的病毒中也观察到了融合后结构的 pH 依赖性稳定性。用 CASV 和相关的金门病毒进行的感染实验支持需要内体酸化的进入机制。我们的结果表明，尽管主要是沙粒病毒科病毒样的，但 CASV 的跨膜亚基与丝状病毒科病毒非常相似。
