Affranchino José L, González Silvia A
Centro de Virología Animal (CEVAN-CONICET), Serrano 669, C1414DEM Buenos Aires, Argentina.
Virology. 2006 Mar 30;347(1):217-25. doi: 10.1016/j.virol.2005.11.032. Epub 2005 Dec 27.
The transmembrane (TM) subunit of the envelope (Env) glycoprotein of the simian immunodeficiency virus (SIV) contains an unusually long cytoplasmic domain of 164 amino acids. Previously, we identified domains in the SIV TM cytoplasmic tail that are necessary for Env incorporation into virions and viral infectivity. In this study, we investigated the relevance to Env function of the highly conserved sequence comprising the immediate C-terminal 19 residues of TM. To this end, small in-frame deletions as well as a premature stop codon mutation were introduced into the coding region for the SIV TM C-terminus. All the mutant Env glycoproteins were expressed, processed and transported to the cell surface in an essentially wild-type manner. Moreover, the ability of the mutant Env proteins to mediate cell-to-cell fusion was similar to or slightly lower than that of the wild-type Env. However, viruses expressing the mutant Env glycoproteins were found to be poorly infectious in single-cycle infectivity assays. Further characterization of the TM mutant viruses revealed that while exhibiting wild-type levels of the TM protein, they contained significantly lower levels of the Env surface (SU) subunit, which is consistent with increased SU shedding from virions after Env incorporation. This phenotype was independent of Gag processing, since genetic inactivation of the viral protease did not increase SU retention by the resulting immature particles. Our findings indicate that deletions at the C-terminus of the SIV Env promote the instability of the SU-TM association on the virion surface and point to an important role for the TM cytoplasmic domain in modulating Env structure.
猴免疫缺陷病毒（SIV）包膜糖蛋白（Env）的跨膜（TM）亚基含有一段长达164个氨基酸的异常长的胞质结构域。此前，我们已鉴定出SIV TM胞质尾中对于Env整合入病毒粒子及病毒感染性所必需的结构域。在本研究中，我们调查了由TM紧邻C末端的19个残基组成的高度保守序列与Env功能的相关性。为此，我们在SIV TM C末端的编码区引入了小的框内缺失以及一个提前终止密码子突变。所有突变的Env糖蛋白均以基本野生型的方式表达、加工并转运至细胞表面。此外，突变的Env蛋白介导细胞间融合的能力与野生型Env相似或略低。然而，在单轮感染性测定中发现，表达突变Env糖蛋白的病毒感染性很差。对TM突变病毒的进一步表征显示，虽然它们呈现出野生型水平的TM蛋白，但它们所含的Env表面（SU）亚基水平显著更低，这与Env整合后病毒粒子中SU脱落增加相一致。这种表型与Gag加工无关，因为病毒蛋白酶的基因失活并未增加由此产生的未成熟颗粒对SU 的保留。我们的研究结果表明，SIV Env C末端的缺失促进了病毒粒子表面SU-TM结合的不稳定性，并指出TM胞质结构域在调节Env结构中起重要作用。
