University of Pennsylvania School of Medicine, Department of Medicine, Division of Infectious Diseases, 502 Johnson Pavilion, Philadelphia, PA 19104-6073, USA.
J Virol. 2010 Nov;84(21):11245-54. doi: 10.1128/JVI.00372-10. Epub 2010 Aug 18.
The vaccinia virus (VACV) complement control protein (VCP) is an immunomodulatory protein that is both secreted from and expressed on the surface of infected cells. Surface expression of VCP occurs though an interaction with the viral transmembrane protein A56 and is dependent on a free N-terminal cysteine of VCP. Although A56 and VCP have been shown to interact in infected cells, the mechanism remains unclear. To investigate if A56 is sufficient for surface expression, we transiently expressed VCP and A56 in eukaryotic cell lines and found that they interact on the cell surface in the absence of other viral proteins. Since A56 contains three extracellular cysteines, we hypothesized that one of the cysteines may be unpaired and could therefore form a disulfide bridge with VCP. To test this, we generated a series of A56 mutants in which each cysteine was mutated to a serine, and we found that mutation of cysteine 162 abrogated VCP cell surface expression. We also tested the ability of other poxvirus complement control proteins to bind to VACV A56. While the smallpox homolog of VCP is able to bind VACV A56, the ectromelia virus (ECTV) VCP homolog is only able to bind the ECTV homolog of A56, indicating that these proteins may have coevolved. Surface expression of poxvirus complement control proteins may have important implications in viral pathogenesis, as a virus that does not express cell surface VCP is attenuated in vivo. This suggests that surface expression of VCP may contribute to poxvirus pathogenesis.
痘苗病毒（VACV）补体控制蛋白（VCP）是一种免疫调节蛋白，既能从感染细胞中分泌出来，也能在感染细胞表面表达。VCP 的表面表达是通过与病毒跨膜蛋白 A56 的相互作用实现的，并且依赖于 VCP 游离的 N 端半胱氨酸。尽管已经表明 A56 和 VCP 在感染细胞中相互作用，但机制仍不清楚。为了研究 A56 是否足以实现表面表达，我们在真核细胞系中转染表达 VCP 和 A56，发现它们在没有其他病毒蛋白的情况下在细胞表面相互作用。由于 A56 含有三个细胞外半胱氨酸，我们假设其中一个半胱氨酸可能未配对，因此可以与 VCP 形成二硫键。为了验证这一点，我们生成了一系列 A56 突变体，其中每个半胱氨酸都突变为丝氨酸，我们发现突变半胱氨酸 162 会使 VCP 丧失细胞表面表达能力。我们还测试了其他痘病毒补体控制蛋白与 VACV A56 的结合能力。虽然 VCP 的天花病毒同源物能够与 VACV A56 结合，但 ECTV 的 VCP 同源物只能与 ECTV 的 A56 同源物结合，这表明这些蛋白可能共同进化。痘病毒补体控制蛋白的表面表达可能对病毒发病机制有重要意义，因为不能表达细胞表面 VCP 的病毒在体内会减弱。这表明 VCP 的表面表达可能有助于痘病毒的发病机制。
