Fig. 5: CHK1/2-mediated ELK3 phosphorylation facilitates ELK3 destruction.

a Dephosphorylation of ELK3 prevent the interaction between ELK3 and SPOP. The cell lysates of HEK293T cells transiently transfected with indicated plasmids were treated with/without λ-phosphatase. The interaction between ELK3 and SPOP was evaluated by IP and WB. b CHK1/2 inhibition suppresses ELK3 destabilization. The cell lysates of HeLa cells treated with 5 μM of indicated inhibitor and CHX (10 μg/ml) for 12 h were used to evaluate the ELK3 protein levels by WB. c CHK1/2 inhibition abolishes ELK3 and SPOP interaction. The cell lysates of HEK293T cells transfected with indicated plasmids treated with 5 μM of indicated inhibitor for 12 h and MG132 (10 μM) for 4 h were used to evaluate the interaction between ELK3 and SPOP by IP and WB. d ELK3 interacts with CHK1 and CHK2. The cell lysates of HEK293T cells transiently transfected with indicated plasmids and treated with MG132 (10 μM) for 4 h were used to evaluate the interaction between ELK3 and CHK1 or CHK2 by IP and WB. e CHK2 facilitates ELK3 ubiquitination by SPOP. cell lysates of HEK293T cells transiently transfected with indicated plasmids and treated with MG132 (10 μM) for 4 h were used to evaluate the ELK3 ubiquitination by IP and WB. f CHK1/2 inhibition abrogates SPOP-mediated ELK3 ubiquitination. The cell lysates of HEK293T cells transiently transfected with indicated plasmids and treated with 5 μM of AZD7762 for 12 h and MG132 (10 μM) for 4 h were used to evaluate the SPOP-mediated ELK3 ubiquitination by IP and WB. g CHK1 knockdown increases ELK3 protein levels. The cell lysates of HeLa cells stably expressing sh-mock or sh-CHK1 or CHK2 were used to evaluate ELK3 protein levels by WB. h CHK1 and CHK2 phosphorylates ELK3. In vitro kinase assay using partially purified His-ELK3 and active CHK1 or CHK2 was conducted. ELK3 phosphorylation by CHK1 or CHK2 was evaluated RxxS/T antibody by WB. i ELK3 deg1 deletion abolishes CHK1- or CHK2-mediated phosphorylation. In vitro kinase assay using partial purified His-ELK3-wt or -∆Deg1 and active CHK1 or CHK2 was conducted. ELK3 phosphorylation by CHK1 or CHK2 was evaluated RxxS/T antibody by WB. j, k CHK2 phosphorylates ELK3 at Ser133 in cell system. The phosphorylation of ELK3 mediated by CHK2 in cell culture (j) and in vitro kinase assay system (k) was evaluated using phos-tag immunoblot analysis. Treatment with AZD7762 (j) and mutation of ELK3 Ser133 to Ala (l) abolished the phosphorylation of ELK3 induced by CHK2. l ELK3 phosphorylation at Ser133 is indispensable to interact with SPOP. ELK3 phosphorylation requirement for the interaction with SPOP was evaluated by IP and western blotting using cell lysates transiently expressing mock, His-ELK3-wt, His-ELK3-S133D, or His-ELK3-S133A.