Bowzard J B, Wills J W, Craven R C
Department of Microbiology and Immunology, The Pennsylvania State University College of Medicine, M. S. Hershey Medical Center, Hershey, Pennsylvania 17033, USA.
J Virol. 2001 Aug;75(15):6850-6. doi: 10.1128/JVI.75.15.6850-6856.2001.
The capsid (CA) protein, the major structural component of retroviruses, forms a shell that encases the ribonucleoprotein complex in the virion core. The most conserved region of CA, approximately 20 amino acids of the major homology region (MHR), lies within the carboxy-terminal domain of the protein. Structural and sequence similarities among CA proteins of retroviruses and the CA-like proteins of hepatitis B virus and various retrotransposons suggest that the MHR is involved in an aspect of replication common to these reverse-transcribing elements. Conservative substitutions in this region of the Rous sarcoma virus protein were lethal due to a severe deficiency in reverse transcription, in spite of the presence of an intact genome and active reverse transcriptase in the particles. This finding suggests that the mutations interfered with normal interactions among these constituents. A total of four genetic suppressors of three lethal MHR mutations have now been identified. All four map to the sequence encoding the CA-spacer peptide (SP) region of Gag. The F167Y mutation in the MHR was fully suppressed by a single amino acid change in the alpha helix immediately downstream of the MHR, a region that forms the major dimer interface in human immunodeficiency virus CA. This finding suggests that the F167Y mutation indirectly interfered with dimerization. The F167Y defect could also be repaired by a second, independent suppressor in the C-terminal SP that was removed from CA during maturation. This single residue change, which increased the rate of SP cleavage, apparently corrected the F167Y defect by modifying the maturation pathway. More surprising was the isolation of suppressors of the R170Q and L171V MHR mutations, which mapped to the N-terminal domain of the CA protein. This finding suggests that the two domains, which in the monomeric protein are separated by a flexible linker, must communicate with each other at some unidentified point in the viral replication cycle.
衣壳（CA）蛋白是逆转录病毒的主要结构成分，形成一个包裹病毒体核心中核糖核蛋白复合物的外壳。CA最保守的区域，即主要同源区域（MHR）的大约20个氨基酸，位于该蛋白的羧基末端结构域内。逆转录病毒的CA蛋白与乙型肝炎病毒的CA样蛋白以及各种逆转座子之间的结构和序列相似性表明，MHR参与了这些逆转录元件共有的复制过程。劳氏肉瘤病毒蛋白该区域的保守性替换是致死性的，因为尽管颗粒中存在完整的基因组和活性逆转录酶，但逆转录严重不足。这一发现表明这些突变干扰了这些成分之间的正常相互作用。目前已经鉴定出三种致死性MHR突变的总共四个遗传抑制子。所有四个都定位于编码Gag的CA间隔肽（SP）区域的序列。MHR中的F167Y突变被MHR下游紧邻的α螺旋中的单个氨基酸变化完全抑制，该区域在人类免疫缺陷病毒CA中形成主要的二聚体界面。这一发现表明F167Y突变间接干扰了二聚化。F167Y缺陷也可以通过成熟过程中从CA去除的C末端SP中的第二个独立抑制子来修复。这一单残基变化增加了SP切割的速率，显然通过改变成熟途径纠正了F167Y缺陷。更令人惊讶的是R170Q和L171V MHR突变的抑制子的分离，它们定位于CA蛋白的N末端结构域。这一发现表明，在单体蛋白中由柔性接头隔开的这两个结构域，在病毒复制周期的某个未确定点必须相互通信。
