Depletion of WRN protein causes RACK1 to activate several protein kinase C isoforms

Werner's syndrome (WS) is a rare autosomal disease characterized by the premature onset of several age-associated pathologies. The protein defective in patients with WS (WRN) is a helicase/exonuclease involved in DNA repair, replication, transcription and telomere maintenance. In this study, we show that a knock down of the WRN protein in normal human fibroblasts induces phosphorylation and activation of several protein kinase C (PKC) enzymes. Using a tandem affinity purification strategy, we found that WRN physically and functionally interacts with receptor for activated C-kinase 1 (RACK1), a highly conserved anchoring protein involved in various biological processes, such as cell growth and proliferation. RACK1 binds strongly to the RQC domain of WRN and weakly to its acidic repeat region. Purified RACK1 has no impact on the helicase activity of WRN, but selectively inhibits WRN exonuclease activity in vitro. Interestingly, knocking down RACK1 increased the cellular frequency of DNA breaks. Depletion of the WRN protein in return caused a fraction of nuclear RACK1 to translocate out of the nucleus to bind and activate PKCdelta and PKCbetaII in the membrane fraction of cells. In contrast, different DNA-damaging treatments known to activate PKCs did not induce RACK1/PKCs association in cells. Overall, our results indicate that a depletion of the WRN protein in normal fibroblasts causes the activation of several PKCs through translocation and association of RACK1 with such kinases.