Functional p53 chimeras containing the Epstein–Barr virus Gly-Ala repeat are protected from Mdm2- and HPV-E6-induced proteolysis

Functional inactivation of the tumor suppressor protein p53 by accelerated ubiquitin/proteasome-dependent proteolysis is a common event in tumor progression. Proteasomal degradation is inhibited by the Gly-Ala repeat (GAr) of the Epstein–Barr virus nuclear antigen-1, which acts as a transferable element on a variety of proteasomal substrates. We demonstrate that p53 chimeras containing GAr domains of different lengths and positions within the protein are protected from proteolysis induced by the ubiquitin ligases murine double minute 2 and E6-associated protein but are still ubiquitinated and retain the capacity to interact with the S5a ubiquitin-binding subunit of the proteasome. The GAr chimeras transactivate p53 target genes, induce cell cycle arrest and apoptosis, and exhibit improved growth inhibitory activity in tumor cells with impaired endogenous p53 activity.

通过泛素（ubiquitin）-蛋白酶体（proteasome）依赖型蛋白水解通路加速肿瘤抑制蛋白p53的功能性失活，是肿瘤进展中的常见事件。爱泼斯坦-巴尔病毒核抗原1（Epstein–Barr virus nuclear antigen-1）的甘氨酸-丙氨酸重复序列（Gly-Ala repeat, GAr）可抑制蛋白酶体介导的蛋白降解，该序列可作为可转移元件，作用于多种蛋白酶体底物。本研究证实，在p53嵌合蛋白中引入不同长度、不同位置的GAr结构域后，其可免受泛素连接酶小鼠双微体2（murine double minute 2, MDM2）与E6相关蛋白（E6-associated protein, E6AP）诱导的蛋白水解，但仍可发生泛素化，且保留与蛋白酶体S5a泛素结合亚基相互作用的能力。此类携带GAr结构域的p53嵌合蛋白可转录激活p53靶基因，诱导细胞周期阻滞与细胞凋亡，并在内源性p53功能受损的肿瘤细胞中展现出更优异的生长抑制活性。