Cullin 4A and 4B ubiquitin ligases interact with γ-tubulin and induce its polyubiquitination.

Regulated polyubiquitination is a key step for controlling protein degradation and maintaining proper balance between the proliferation of normal and uncontrolled cells. Addition of ubiquitin to the proteins by E3 ubiquitin ligases targets them for degradation by the 26S proteosome machinery. Discrepancies in ubiquitination and/or proteosome degradation might lead to multiple genetic disorders in humans. It is reported that CUL1 and BRCA1 ubiquitin ligases localize on centrosome region and regulate the centrosome duplication cycle for genomic stability. In the current study, we predicted the possible interaction of E3 ubiquitin ligase CUL4A complex with γ-tubulin, a centrosome-specific protein, using bioinformatic protein-protein docking analysis. We also confirmed their interaction by performing co-immunoprecipitation studies using endogenous CUL4A/B and stable cell lines that overexpress Flag-CUL4A or Flag-CUL4B. We additionally noted that the γ-tubulin was polyubiquitinated by CUL4A or 4B immune complex indicating that CUL4A or CUL4B may regulate the stability of γ-tubulin. Furthermore, the inhibition of proteosomal degradation pathway using MG132 or LLNV drugs resulted in accumulation and co-localization of CUL4A with γ-tubulin in the centrosome region. Overall, our observation has identified γ-tubulin as a novel target for E3 ubiquitin ligase CUL4 complex, and might lead to the establishment of a unique mechanism for controlling centrosome stability.