Department of Molecular Biology and Biochemistry and Cancer Research Institute, University of California, Irvine, CA 92697-3905, USA.
Retrovirology. 2012 Jul 24;9:58. doi: 10.1186/1742-4690-9-58.
One of the unique features of gammaretroviruses is that they contain an additional extended form of Gag, glyco-gag, which initiates in the leader sequence. MuLV glyco-gag, gPr80Gag, promotes retrovirus replication and disease progression. Although virtually all infectious MuLVs encode glyco-gag, XMRV (xenotropic murine leukemia virus-related virus) lacks the classical gPr80Gag sequence. We examined XMRV to determine if its leader sequence contains glyco-gag activity, whether the presence of conventional gPr80Gag affects replication of XMRV, and we describe the evolution of glyco-gag-deficient MuLVs in Mus.
We introduced several mutations disrupting two putative but noncanonical glyco-gag proteins in the leader sequence region in XMRV and found that those mutations did not affect virus release nor susceptibility to the antiviral activity of hA3G (human APOBEC3G). A chimeric XMRV encoding the Moloney MuLV (M-MuLV) leader sequence (MXMRV) demonstrated that M-MuLV glyco-gag facilitated MXMRV release and increased infectivity. Infectivity assays with several cell lines showed that glyco-gag increases XMRV infectivity in all cell lines tested, but the level of this increase varies in different cell lines. Because MuLV glyco-gag counteracts mouse APOBEC3, we investigated whether M-MuLV glyco-gag enhances XMRV infection by counteracting human APOBEC3. Comparison of hAPOBEC3 isoforms expressed in different cell lines indicated that hA3B was the most likely candidate for a restrictive hA3. However over-expression of hA3B showed no enhanced restriction of infection by XMRV compared to MXMRV. Endogenous MuLVs in the sequenced mouse genome were screened for canonical glyco-gag, which was identified in two clades of xenotropic MuLVs (X-MuLVs) and ecotropic MuLVs, but not in other X-MuLVs or in any polytropic MuLVs.
M-MuLV glyco-gag facilitates XMRV replication, and the leader sequence region in XMRV does not encode proteins equivalent to M-MuLV glyco-gag. The fact that the ability of glyco-gag to enhance XMRV infection varies in different cell lines suggests a glyco-gag sensitive restrictive factor that further reduces XMRV infectivity. The M-MuLV glyco-gag enhancement for XMRV replication is through a hAPOBEC3 independent mechanism. The absence of glyco-gag in MuLVs carried by western European mice suggests that loss of this sequence is a relatively recent event with limited subspecies distribution.
γ逆转录病毒的一个独特特征是它们包含额外的扩展形式的 Gag，糖基化 Gag，它起始于先导序列。MuLV 糖基化 Gag，gPr80Gag，促进逆转录病毒复制和疾病进展。尽管几乎所有传染性 MuLV 都编码糖基化 Gag，但 XMRV（异嗜性鼠白血病病毒相关病毒）缺乏经典的 gPr80Gag 序列。我们研究了 XMRV，以确定其先导序列是否含有糖基化 Gag 活性，常规 gPr80Gag 的存在是否影响 XMRV 的复制，并且我们描述了 Mus 中糖基化 Gag 缺失 MuLV 的进化。
我们引入了几种突变，破坏了 XMRV 先导序列区域中两个假定但非典型的糖基化 Gag 蛋白，发现这些突变不会影响病毒释放，也不会影响 hA3G（人 APOBEC3G）的抗病毒活性。一种编码莫洛尼 MuLV（M-MuLV）先导序列的嵌合 XMRV（MXMRV）表明，M-MuLV 糖基化 Gag 促进了 MXMRV 的释放并增加了感染性。用几种细胞系进行的感染性测定表明，糖基化 Gag 增加了所有测试细胞系中 XMRV 的感染性，但在不同细胞系中增加的程度不同。由于 MuLV 糖基化 Gag 会对抗小鼠 APOBEC3，我们研究了 M-MuLV 糖基化 Gag 是否通过对抗人 APOBEC3 来增强 XMRV 感染。不同细胞系中表达的 hAPOBEC3 同工型的比较表明，hA3B 是最有可能的限制性 hA3 候选者。然而，与 MXMRV 相比，hA3B 的过表达并未显示出对 XMRV 感染的增强限制。对测序的小鼠基因组中的内源性 MuLV 进行了筛选，以寻找典型的糖基化 Gag，在异嗜性 MuLV（X-MuLV）和嗜性 MuLV 中发现了这种糖基化 Gag，但在其他 X-MuLV 或任何多嗜性 MuLV 中都没有发现。
M-MuLV 糖基化 Gag 促进了 XMRV 的复制，而 XMRV 的先导序列区域不编码相当于 M-MuLV 糖基化 Gag 的蛋白质。糖基化 Gag 增强 XMRV 感染的能力在不同细胞系中存在差异，这表明存在一种糖基化 Gag 敏感的限制因子，进一步降低了 XMRV 的感染性。M-MuLV 糖基化 Gag 对 XMRV 复制的增强作用是通过 hAPOBEC3 独立的机制。西欧小鼠携带的 MuLV 中缺乏糖基化 Gag 表明，这种序列的缺失是一个相对较新的事件，其分布范围有限。
