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The nuclear–cytoplasmic shuttling protein RBCC protein interacting with protein kinase C1 (RBCK1) possesses transcriptional and ubiquitin ligase activities. We have recently reported that RBCC protein interacting with protein kinase C2 (RBCK2), a RING–in-between-RING fingers domain-lacking splice variant of RBCK1, lacks transcriptional activity, but rather represses the RBCK1-mediated transcriptional activity as a cytoplasmic tethering protein for RBCK1. | ỊFEBS Journal Accumulation of polyubiquitinated proteins by overexpression of RBCC protein interacting with protein kinase C2 a splice variant of ubiquitin ligase RBCC protein interacting with protein kinase C1 Nobuo Yoshimoto1 2 Kenji Tatematsu1 Toshihide Okajima3 Katsuyuki Tanizawa1 and Shun ichi Kuroda1 2 1 Department of StructuralMolecular Biology Institute of Scientific and IndustrialResearch Osaka University Japan 2 Laboratory of IndustrialBiosciences Graduate Schoolof BioagriculturalSciences Nagoya University Japan 3 Department of Nanobiology Institute of Scientific and IndustrialResearch Osaka University Japan Keywords 26S proteasome RING-IBR S5a splice variant ubiquitin Correspondence K. Tatematsu and S. Kuroda Laboratory of IndustrialBiosciences Graduate Schoolof Bioagricultural Sciences Nagoya University Furo-cho Chikusa Nagoya 464-8601 Japan Fax 81 52 789 5227 Tel 81 52 789 5227 E-mail ktatematsu@ucsd.edu and skuroda@agr.nagoya-u.ac.jp Received 10 June 2009 revised 26 August 2009 accepted 3 September 2009 doi 10.1111 j.1742-4658.2009.07350.x The nuclear-cytoplasmic shuttling protein RBCC protein interacting with protein kinase C1 RBCK1 possesses transcriptional and ubiquitin ligase activities. We have recently reported that RBCC protein interacting with protein kinase C2 RBCK2 a RING-in-between-RING fingers domainlacking splice variant of RBCK1 lacks transcriptional activity but rather represses the RBCKI-mediated transcriptional activity as a cytoplasmic tethering protein for RBCK1. In this study we have found that RBCK2 overexpressed in human embryonic kidney 293 cells interacts with the polyubiquitin chain and the polyubiquitin-interacting subunit S5a and significantly increases the intracellular amount of polyubiquitinated proteins. These results strongly suggested that RBCK2 functions as an adaptor protein for the polyubiquitinated protein and the S5a subunit in 26S proteasome through its novel zinc finger motif and ubiquitin-like domain .