Pavio Nicole, Taylor Deborah R, Lai Michael M C
Howard Hughes Medical Institute, Keck School of Medicine, University of Southern California, Los Angeles, California 90033, USA.
J Virol. 2002 Feb;76(3):1265-72. doi: 10.1128/jvi.76.3.1265-1272.2002.
The hepatitis C virus (HCV) envelope protein E2 has been shown to accumulate in the lumen of the endoplasmic reticulum (ER) as a properly folded glycoprotein as well as large aggregates of misfolded proteins. In the present study, we have identified an additional unglycosylated species, with an apparent molecular mass of 38 kDa (E2-p38). In contrast to the glycosylated E2, E2-p38 is significantly less stable and is degraded through the proteasome pathway. Correspondingly, E2-p38 is found to be ubiquitinated. E2-p38 is localized mostly in the cytosol, in contrast to the glycosylated form, which is exclusively membrane associated. Alpha interferon (IFN-alpha) treatment or overexpression of the double-stranded RNA-activated protein kinase (PKR) significantly increased the stability of E2-p38, consistent with a previous report (D. R. Taylor, S. T. Shi, P. R. Romano, G. N. Barber, and M. M. Lai, Science 285:107-110, 1999) that E2 interacts with PKR and inhibits its kinase activity. Direct interaction between PKR and E2-p38, but not the glycosylated form of E2, was also observed. These results show that E2-p38 is the form of E2 that interacts with PKR in the cytosol and may contribute to the resistance of HCV to IFN-alpha. Thus, an ER protein can exist in the cytosol as an unglycosylated species and impair cellular functions.
丙型肝炎病毒（HCV）包膜蛋白E2已被证明以内质网（ER）腔内正确折叠的糖蛋白以及错误折叠蛋白的大聚集体形式积累。在本研究中，我们鉴定出了另一种未糖基化的形式，其表观分子量为38 kDa（E2-p38）。与糖基化的E2不同，E2-p38稳定性明显较低，并通过蛋白酶体途径降解。相应地，发现E2-p38被泛素化。与仅与膜相关的糖基化形式不同，E2-p38主要定位于细胞质中。α干扰素（IFN-α）处理或双链RNA激活蛋白激酶（PKR）的过表达显著提高了E2-p38的稳定性，这与之前的一份报告一致（D.R.泰勒、S.T.施、P.R.罗曼诺、G.N.巴伯和M.M.赖，《科学》285:107-110，1999），即E2与PKR相互作用并抑制其激酶活性。还观察到PKR与E2-p38之间有直接相互作用，但与E2的糖基化形式没有相互作用。这些结果表明，E2-p38是E2在细胞质中与PKR相互作用的形式，可能有助于HCV对IFN-α的抗性。因此，一种内质网蛋白可以以未糖基化的形式存在于细胞质中并损害细胞功能。
