Figure 2. EBNA-3C accelerates cell cycle activities by overriding multiple checkpoints.

(A) EBNA-3C forms a complex with cyclin D1/CDK6 and enhances its stability through inhibiting both polyubiquitination and GSK3β-mediated phosphorylation. EBNA-3C further enhances the kinase activity of the the cyclin D1/CDK6 complex and recruits its activity to facilitate the ubiquitination and subsequent degradation of the hyperphosphorylated form of pRb, which, in turn, releases E2F transcription factor from an inhibitory constraint and enables the expression of genes required for G1–S-phase transition. In addition, EBNA-3C coupled with CtBP negatively regulates transcriptional expression of p16^INK4A, which is a specific CDK inhibitor of the cyclin D1/CDK6 complex. EBNA-3C was also shown to form complexes with cyclin D2 and D3. However, whether EBNA-3C has any role in regulating their activity during the cell cycle is largely unknown. (B) EBNA-3C also forms a direct complex with cyclin A/CDK2, blocks p27^KIP1-mediated suppression and subsequently increases its kinase activity to enhance pRb phosphorylation status. EBNA-3C recruits SCF^skp2 E3 ligase activity to both pRb and p27^KIP1 for facilitating their ubiquitin–proteasome-mediated degradation. Altogether, these activities contribute to both the G1–S and S–G2 transition of the cell cycle. (C) By regulating both Chk2 and p53-mediated activities, EBNA-3C indirectly enhances kinase activity of the cyclin B1/CDC2 complex. EBNA-3C blocks p53-dependent transcriptional activation of p21^CIP1, which negatively regulates cyclin B1 and activity. EBNA-3C directly interacts with Chk2, which results in the inactivation of Cdc25c through phosphorylation at S₂₁₆ sequestration in the cytoplasm. The resulting effect leads to the kinase activation of cyclin B/CDC2 and subsequent cell cycle progression through the G2/M stage.