MUF1, a novel Elongin BC-interacting leucine-rich repeat protein that can assemble with Cul5 and Rbx1 to reconstitute a ubiquitin ligase

The heterodimeric. Elongin BC complex has been shown to interact in vitro a nd in mammalian cells with a conserved BC-box motif found in a growing numb er of proteins including RNA polymerase Il elongation factor Elongin A, SOC S-box proteins, and the von Hippel-Lindau (VHL) tumor suppressor protein. R ecently, the VHL-EIongin BC complex was found to interact with a module com posed of Cullin family member Cul2 and RING-H2 finger protein Rbx1 to recon stitute a novel E3 ubiquitin ligase that activates ubiquitylation by the E2 ubiquitin-conjugating enzymes Ubc5 and Cdc34. In the context of: the VEL u biquitin ligase, Elongin BC functions as an adaptor that links the VHL prot ein to the CuI2/Rbx1 module, raising the possibility that the Elongin BC co mplex could function as an integral component of a larger family of E3 ubiq uitin ligases by linking alternative BC-box proteins to Cullin/Rbx1 modules . In this report, we describe identification and purification from rat live r of a novel leucine-rich repeat-containing BO-box protein, MUF1, which we demonstrate is capable of assembling, with a Cullin/Rbx1 module containing the Cullin family member Cul5 to reconstitute ubiquitin ligase activity. In addition, we show that the additional BC-box, proteins Elongin A, SOCS1, a nd WSB1 are also capable of assembling with the Cul5/Rbx1. module to recons titute potential ubiquitin ligases. Taken together, our findings identify M UF1 as a new member of the BO-box family of proteins, and they predict the existence of a larger family of Elongin BC-based E3 ubiquitin ligases.