Requirement of CRTC1 coactivator for hepatitis B virus transcription

Tang, Hei-Man Vincent; Gao, Wei-Wei; Chan, Chi-Ping; Cheng, Yun; Chaudhary, Vidyanath; Deng, Jian-Jun; Yuen, Kit-San; Wong, Chun-Ming; Ng, Irene Oi-Lin; Kok, Kin-Hang; Zhou, Jie; Jin, Dong-Yan

To shed mechanistic light on the mutual stabilization of HBx and CRTC1, we employed proteosome inhibitor MG132 and a lysine-free (K0) mutant of ubiquitin (26) . The K0 mutant of ubiquitin prevents polyubiquitination and subsequent proteasome-mediated degradation, but allows mono-and multi-ubiquitination, which might be linked to lysosome-dependent degradation (36, 37) . If mutual stabilization of HBx and CRTC1 is mediated solely through polyubiquitination and proteosome, their levels should be unchanged or increased in the presence of MG132 or ubiquitin-K0. Interestingly, the steady-state levels of CRTC1 and HBx decreased only slightly when the cells were treated with MG132 ( Figure 7F , lane 4 compared to 3), implicating a role for proteosome in the stabilization effect. However, the expression of ubiquitin-K0 abrogated this stabilization effect almost completely ( Figure 7F , lane 5 compared to 1-4). Although there was no simple interpretation to this result, one possibility is that mono-or multiubiquitination of HBx and CRTC1 might promote their proteosomal degradation. Plausibly, the stability of HBx and CRTC1 might be governed by multiple mechanisms involving ubiquitination, proteosome and lysosome.