Levitskaya J, Sharipo A, Leonchiks A, Ciechanover A, Masucci M G
Microbiology and Tumor Biology Center, Karolinska Institute, Box 280, S-171 77 Stockholm, Sweden.
Proc Natl Acad Sci U S A. 1997 Nov 11;94(23):12616-21. doi: 10.1073/pnas.94.23.12616.
The Epstein-Barr virus (EBV) encoded nuclear antigen (EBNA) 1 is expressed in latently infected B lymphocytes that persist for life in healthy virus carriers and is the only viral protein regularly detected in all EBV associated malignancies. The Gly-Ala repeat domain of EBNA1 was shown to inhibit in cis the presentation of major histocompatibility complex (MHC) class I restricted cytotoxic T cell epitopes from EBNA4. It appears that the majority of antigens presented via the MHC I pathway are subject to ATP-dependent ubiquitination and degradation by the proteasome. We have investigated the influence of the repeat on this process by comparing the degradation of EBNA1, EBNA4, and Gly-Ala containing EBNA4 chimeras in a cell-free system. EBNA4 was efficiently degraded in an ATP/ubiquitin/proteasome-dependent fashion whereas EBNA1 was resistant to degradation. Processing of EBNA1 was restored by deletion of the Gly-Ala domain whereas insertion of Gly-Ala repeats of various lengths and in different positions prevented the degradation of EBNA4 without appreciable effect on ubiquitination. Inhibition was also achieved by insertion of a Pro-Ala coding sequence. The results suggest that the repeat may affect MHC I restricted responses by inhibiting antigen processing via the ubiquitin/proteasome pathway. The presence of regularly interspersed Ala residues appears to be important for the effect.
爱泼斯坦-巴尔病毒（EBV）编码的核抗原（EBNA）1在潜伏感染的B淋巴细胞中表达，这些细胞在健康病毒携带者体内终生存在，并且是在所有EBV相关恶性肿瘤中经常检测到的唯一病毒蛋白。EBNA1的甘氨酸-丙氨酸重复结构域已被证明可顺式抑制主要组织相容性复合体（MHC）I类限制性细胞毒性T细胞表位从EBNA4的呈递。似乎通过MHC I途径呈递的大多数抗原都要经过蛋白酶体依赖ATP的泛素化和降解。我们通过在无细胞系统中比较EBNA1、EBNA4和含甘氨酸-丙氨酸的EBNA4嵌合体的降解情况，研究了该重复序列对这一过程的影响。EBNA4以ATP/泛素/蛋白酶体依赖的方式有效降解，而EBNA1对降解具有抗性。通过缺失甘氨酸-丙氨酸结构域可恢复EBNA1的加工过程，而在不同位置插入不同长度的甘氨酸-丙氨酸重复序列可阻止EBNA4的降解，而对泛素化没有明显影响。插入脯氨酸-丙氨酸编码序列也可实现抑制作用。结果表明，该重复序列可能通过抑制泛素/蛋白酶体途径的抗原加工来影响MHC I限制性反应。规则间隔的丙氨酸残基的存在似乎对该效应很重要。
