Tatematsu, K., Yoshimoto, N., Okajima, T., Tanizawa, K., and Kuroda, S.
Notes: RBCK1 is a RING-IBR (ring in between ring fingers) protein previously shown to have transcriptional activity and to bind protein Kinase Cβ. This paper demonstrates that RBCK1 also possesses ubiquitin ligase E3 activity. Both FLAG-tag and HaloTag® labeled proteins were used to demonstrate this activity. To demonstrate the interaction between RBCK1 and ubiquitinated proteins, HaloTag®-ubiquitin and FLAG-RBCK1 were coexpressed in HEK293 cells in the presence or absence of a proteasome inhibitor. The anti-FLAG immunoprecipitates isolated from these cells were analyzed by SDS-PAGE and Western blotting using anti-HaloTag® and anti-FLAG antibodies and self-ubiquitination of RBCK1 was demonstrated. RBCK2, a splice variant of RBCK1 lacking the RING domain showed no self ubiquitination activity but was demonstrated to interact with RBCK1 in vivo and in vitro, and to inhibit the self-ubiquitination activity of RBCK1 in a FLAG-tag assay. Pulse-chase experiments, using HEK293 cells expressing HaloTag®-RBCK1 with or without RBCK2 and treated with HaloTag®-TMR ligand, were used to show that the half-life of RBCK1 was extended by overexpression of RBCK2. (3874)