The xeroderma pigmentosum group E gene product DDB2 is a specific target of cullin 4A in mammalian cells

The damaged-DNA binding protein DDB consists of two subunits, DDB1 (127 kDa ) and DDB2 (48 kDa). Mutations in the DDB2 subunit have been detected in pa tients suffering from the repair deficiency disease xeroderma pigmentosum ( group E). In addition, recent studies suggested a role for DDB2 in global g enomic repair. DDB2 also exhibits transcriptional activity. We showed that expression of DDB1 and DDB2 stimulated the activity of the cell cycle regul atory transcription factor E2F1. Here we show that DDB2 is a cell cycle-reg ulated protein. It is present at a low level in growth-arrested primary fib roblasts, and after release the level peaks at the G(1)/S boundary. The cel l cycle regulation of DDB2 involves posttranscriptional mechanisms. Moreove r, we find that an inhibitor of 26S proteasome increases the level of DDB2, suggesting that it is regulated by the ubiquitin-proteasome pathway. Our p revious study indicated that the cullin family protein Cul-4A associates wi th the DDB2 subunit. Because cullins are involved in the ubiquitin-proteaso me pathway, we investigated the role of Cul-4A in regulating DDB2. Here we show that DDB2 is a specific target of Cul-4A. Coexpression of Cul-4A, but not Cul-1 or other highly related cullins, increases the ubiquitination and the decay rate of DDB2. A naturally occurring mutant of DDB2 (2RO), which does not bind Cul-4A, is not affected by coexpression of Cul-4A. Studies pr esented here identify a specific function of the Cul-4A gene, which is ampl ified and overexpressed in breast cancers.