RBCC protein interacting with C-kinase (RBCK 1) is a transcription factor belonging to the RING-IBR protein family and has been shown to shuttle between the nucleus and cytoplasm, possessing both the nuclear export and localization signals within its amino acid sequence. RBCK2, lacking the C-terminal half of RBCK I including the RING-IBR domain, has also been identified as an alternative splice variant of RBCK1. RBCK2 shows no transcriptional activity and instead it represses the transcriptional activity of RBCK]. Here, we show that RBCK2 is present usually in the cytoplasm containing two Leu-rich regions that presumably serve as a nuclear export signal (NES). Moreover, an NES-disrupted RBCK1 that is mostly localized within the nucleus is translocated to the cytoplasm when coexpressed with RBCK2, suggesting that RBCK2 serves as a cytoplasmic tethering protein for RBCK L We propose a novel and general function of RING-lacking splice variants of RING proteins to control the intracellular localization and functions of the parental RING proteins by forming a hetero-oligomeric complex. (c) 2005 Elsevier Inc. All rights reserved.