Fan H, Chute H, Chao E, Feuerman M
Proc Natl Acad Sci U S A. 1983 Oct;80(19):5965-9. doi: 10.1073/pnas.80.19.5965.
Murine leukemia virus (MuLV) encodes two independent pathways for expression of the gag gene. One pathway results in processing and cleavage of the precursor Pr65gag to yield the internal capsid proteins of the virion and is analogous to gag polyprotein precursors for all classes of retroviruses. The other pathway, which is not encoded by several other classes of retroviruses, begins with a glycosylated polyprotein gPr80gag . gPr80gag is synthesized independently of Pr65gag; it contains Pr65gag peptides and additional amino-terminal protein. It is modified by further addition of carbohydrate, exported to the cell surface, and released from the cell but does not appear in virus particles. To investigate the role of glycosylated gag in MuLV infection, two mutants of Moloney MuLV (M-MuLV) deficient for synthesis of gPr80gag but able to synthesize Pr65gag were constructed. The mutants were obtained by substitution into a molecular clone of M-MuLV DNA by DNA from two acutely transforming viruses, Ableson MuLV (Ab-MuLV) and Moloney murine sarcoma virus (M-MSV). Both Ab-MuLV and M-MSV are derived from M-MuLV and they express M-MuLV gag sequences, but some strains do not synthesize glycosylated gag protein. For Ab-MuLV, a 177-base-pair Pst I fragment from the P90 strain containing the initiation codon for Pr65gag was substituted for the equivalent fragment in M-MuLV DNA. For M-MSV, 1.5 kilobases at the 5' end of the genome was substituted. Transfection of the recombined DNAs onto NIH-3T3 cells produced infectious M-MuLV, although the infected cells did not produce gPr80gag. Therefore glycosylated gag is not absolutely required for MuLV replication. Deletion of the glycosylated gag pathway did not significantly reduce the level of virus production, although a minor difference in XC plaque morphology was observed.
鼠白血病病毒（MuLV）编码两条独立的gag基因表达途径。一条途径导致前体Pr65gag的加工和切割，产生病毒体的内部衣壳蛋白，这与所有类别的逆转录病毒的gag多蛋白前体类似。另一条途径并非其他几类逆转录病毒所编码，它起始于一种糖基化多蛋白gPr80gag。gPr80gag独立于Pr65gag合成；它包含Pr65gag肽段和额外的氨基末端蛋白。它通过进一步添加碳水化合物进行修饰，转运至细胞表面并从细胞中释放，但不出现在病毒颗粒中。为了研究糖基化gag在MuLV感染中的作用，构建了两个莫洛尼MuLV（M-MuLV）突变体，它们缺乏gPr80gag的合成能力，但能够合成Pr65gag。这些突变体是通过用来自两种急性转化病毒——阿贝尔森MuLV（Ab-MuLV）和莫洛尼鼠肉瘤病毒（M-MSV）的DNA替换M-MuLV DNA的分子克隆而获得的。Ab-MuLV和M-MSV均源自M-MuLV，它们表达M-MuLV gag序列，但一些毒株不合成糖基化gag蛋白。对于Ab-MuLV，用来自P90毒株的包含Pr65gag起始密码子的177个碱基对的Pst I片段替换M-MuLV DNA中的等效片段。对于M-MSV，则替换基因组5'端的1.5千碱基。将重组DNA转染到NIH-3T3细胞上产生了有感染性的M-MuLV，尽管受感染细胞不产生gPr80gag。因此，糖基化gag并非MuLV复制所绝对必需的。糖基化gag途径的缺失并未显著降低病毒产生水平，尽管观察到XC噬斑形态存在微小差异。
