Kim Jung-Hwan, Sohn Sook-Young, Benedict Yen T S, Ahn Byung-Yoon
School of Life Sciences & Biotechnology, Korea University, Anamdong 5-1, Sungbuk, Seoul 136-701, Republic of Korea.
Biochem Biophys Res Commun. 2008 Feb 22;366(4):1036-42. doi: 10.1016/j.bbrc.2007.12.070. Epub 2007 Dec 26.
The hepatitis B virus X protein (HBX) plays key regulatory roles in viral replication and the development of hepatocellular carcinoma. HBX is an unstable protein; its instability is attributed to rapid degradation through the ubiquitin-proteasome pathway. Here, we show that the middle and carboxyl-terminal domains of HBX, independently fused to GFP, render the recombinant proteins susceptible to proteasomal degradation, while the amino-terminal domain has little effect on the ubiquitination or stability of HBX. Mutation of any single or combination of up to five of six lysine residues, all located in the middle and carboxyl-terminal domain, did not prevent HBX from being ubiquitinated, ruling out any specific lysine as the sole site of ubiquitination. Surprisingly, HBX in which all six lysines were mutated and showed no evidence of ubiquitination, was still susceptible to proteasomal degradation. These results suggest that both ubiquitin-dependent and -independent proteasomal degradation processes are operative in HBX turnover.
乙肝病毒X蛋白（HBX）在病毒复制和肝细胞癌的发展过程中发挥着关键的调节作用。HBX是一种不稳定的蛋白质；其不稳定性归因于通过泛素-蛋白酶体途径的快速降解。在此，我们表明，独立与绿色荧光蛋白（GFP）融合的HBX的中间结构域和羧基末端结构域，使重组蛋白易于被蛋白酶体降解，而氨基末端结构域对HBX的泛素化或稳定性影响很小。六个赖氨酸残基中的任何一个或最多五个的组合发生突变，所有这些赖氨酸残基都位于中间结构域和羧基末端结构域，都不能阻止HBX被泛素化，排除了任何特定赖氨酸作为唯一泛素化位点的可能性。令人惊讶的是，所有六个赖氨酸都发生突变且没有泛素化证据的HBX，仍然易于被蛋白酶体降解。这些结果表明，泛素依赖性和非依赖性蛋白酶体降解过程在HBX的周转中都起作用。
