Gu J, Parthasarathi S, Varela-Echavarría A, Ron Y, Dougherty J P
Department of Molecular Genetics and Microbiology, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, Piscataway 08854-5635.
Virology. 1995 Feb 1;206(2):885-93. doi: 10.1006/viro.1995.1011.
The envelope glycoprotein complex is composed of two polypeptides, an external heavily glycosylated polypeptide (SU) and a membrane-spanning protein (TM). Together they form a heterodimer on the surface of the virion. These proteins are synthesized in the form of a polyprotein precursor which is glycosylated and proteolytically processed during its maturation in the secretory pathway. A highly conserved stretch of four amino acids, CWLC, has been identified in most known oncoretroviral SU proteins, about two-thirds of the distance from the amino terminus. To study the significance of this sequence for the structure and/or function of SU, cysteine to serine mutations were made in reticuloendotheliosis virus strain A. Initial studies showed that substitution of either one or both cysteines resulted in the production of noninfectious virus. Furthermore, immunoprecipitations and pulse-chase analysis demonstrated that the mutants yielded envelope polyprotein precursors which were stable. However, the polyprotein precursors were not proteolytically processed into SU and TM, and immunoprecipitations indicate that the immature polyproteins form aggregates, suggesting that the mutations interfere with proper folding. Although not proteolytically processed, at least one of the mutant glycoproteins appeared to be efficiently transported to the cell surface. These studies indicate that changing either cysteine residue abrogates viral infectivity by affecting folding, inhibiting normal maturation of the envelope glycoproteins.
包膜糖蛋白复合体由两种多肽组成，一种是外部高度糖基化的多肽（SU），另一种是跨膜蛋白（TM）。它们共同在病毒粒子表面形成异二聚体。这些蛋白质以多蛋白前体的形式合成，在其于分泌途径中成熟的过程中进行糖基化和蛋白水解加工。在大多数已知的致肿瘤逆转录病毒SU蛋白中，已鉴定出一段高度保守的四个氨基酸序列CWLC，位于距氨基末端约三分之二的位置。为了研究该序列对SU结构和/或功能的重要性，在网状内皮增生症病毒A株中对该序列中的半胱氨酸进行了丝氨酸突变。初步研究表明，一个或两个半胱氨酸被取代都会导致产生无感染性的病毒。此外，免疫沉淀和脉冲追踪分析表明，这些突变体产生的包膜多蛋白前体是稳定的。然而，多蛋白前体未被蛋白水解加工成SU和TM，免疫沉淀表明未成熟的多蛋白形成聚集体，这表明突变干扰了正确折叠。尽管未进行蛋白水解加工，但至少有一种突变糖蛋白似乎能有效地转运到细胞表面。这些研究表明，改变任一半胱氨酸残基会通过影响折叠来消除病毒感染性，从而抑制包膜糖蛋白的正常成熟。
