Burkhart Michael D, D'Agostino Paul, Kayman Samuel C, Pinter Abraham
Public Health Research Institute, 225 Warren Street, Newark, NJ 07103-3506, USA.
J Virol. 2005 Jun;79(12):7868-76. doi: 10.1128/JVI.79.12.7868-7876.2005.
A role for the C-terminal domain (CTD) of murine leukemia virus (MuLV) Env protein in viral fusion was indicated by the potent inhibition of MuLV-induced fusion, but not receptor binding, by two rat monoclonal antibodies (MAbs) specific for epitopes in the CTD. Although these two MAbs, 35/56 and 83A25, have very different patterns of reactivity with viral isolates, determinants of both epitopes were mapped to the last C-terminal disulfide-bonded loop of SU (loop 10), and residues in this loop responsible for the different specificities of these MAbs were identified. Both MAbs reacted with a minor fraction of a truncated SU fragment terminating four residues after loop 10, indicating that while the deleted C-terminal residues were not part of these epitopes, they promoted their formation. Neither MAb recognized the loop 10 region expressed in isolated form, suggesting that these epitopes were not completely localized within loop 10 but required additional sequences located N terminal to the loop. Direct support for a role for loop 10 in fusion was provided by the demonstration that Env mutants containing an extra serine or threonine residue between the second and third positions of the loop were highly attenuated for infectivity and defective in fusion assays, despite wild-type levels of expression, processing, and receptor binding. Other mutations at positions 1 to 3 of loop 10 inhibited processing of the gPr80 precursor protein or led to increased shedding of SU, suggesting that loop 10 also affects Env folding and the stability of the interaction between SU and TM.
鼠白血病病毒（MuLV）Env蛋白的C末端结构域（CTD）在病毒融合中所起的作用，是由两种对CTD中表位具有特异性的大鼠单克隆抗体（MAb）有力抑制MuLV诱导的融合，但不抑制受体结合这一现象所表明的。尽管这两种MAb（35/56和83A25）与病毒分离株的反应模式截然不同，但两个表位的决定簇都被定位到SU的最后一个C末端二硫键结合环（环10），并且鉴定出了该环中负责这些MAb不同特异性的残基。两种MAb都与在环10之后四个残基处终止的截短SU片段的一小部分发生反应，这表明虽然缺失的C末端残基不是这些表位的一部分，但它们促进了表位的形成。两种MAb都不识别以分离形式表达的环10区域，这表明这些表位并不完全定位于环10内，而是需要位于该环N末端的额外序列。环10在融合中起作用的直接证据是，有研究表明，尽管Env突变体在表达、加工和受体结合方面处于野生型水平，但在环的第二个和第三个位置之间含有一个额外丝氨酸或苏氨酸残基的Env突变体，其感染性高度减弱，并且在融合试验中存在缺陷。环10第1至3位的其他突变抑制了gPr80前体蛋白的加工或导致SU的脱落增加，这表明环10也影响Env的折叠以及SU与TM之间相互作用的稳定性。
