The Elongin B ubiquitin homology domain - Identification of Elongin B sequences important for interaction with Elongin C

Mammalian Elongin B is a 118-amino acid protein composed of an 84-amino aci d amino-terminal ubiquitin-like domain and a 34-amino acid carboxyl-termina l tail. Elongin B is found in cells as a subunit of the heterodimeric Elong in BC complex, which was originally identified as a positive regulator of R NA polymerase II elongation factor Elongin A and subsequently as a componen t of the multiprotein von Hippel-Lindau tumor suppressor and suppressor of cytokine signaling complexes, As part of our effort to understand how the E longin BC complex regulates the activity of Elongin A, we are characterizin g Elongin B functional domains, In this report, we show that the Elongin B ubiquitin-like domain is necessary and sufficient for interaction with Elon gin C and for positive regulation of Elongin A transcriptional activity. In addition, by site-directed mutagenesis of the Elongin B ubiquitin-like dom ain, we identify a short Elongin B region that is important for its interac tion with Elongin C, Finally, we observe that both the ubiquitin-like domai n and carboxyl-terminal tail are conserved in Drosophila melanogaster and C aenorhabditis elegans Elongin B homologs that efficiently substitute for ma mmalian Elongin B in reconstitution of the transcriptionally active Elongin ABC complex, suggesting that the carboxyl-terminal tail performs an additi onal function not detected in our assays.