The CROC1 isoforms, also known as UBE2V1, show sequence similarity to ubiquitin-conjugating enzymes (UBCs, or E2s) but lack the conserved cysteine residue critical to catalytic activity of E2s.1 Northern blot analysis detected approximately 2.1- and 2.5-kb CROC1 transcripts in all human tissues examined, with the highest levels in brain, skeletal muscle, and kidney. Partial human intestinal epithelial cell cDNAs have been isolated containing the 3-primecoding sequence and 3-prime untranslated region of UBE2V1, also called UEV1.2 UEV1 does not have ubiquitin-conjugating activity in vitro. UEV1 transcripts are downregulated upon differentiation of a colon carcinoma cell line.1 Constitutive expression of exogenous UEV1 protein in these colon carcinoma cells inhibits their capacity to differentiate upon confluence and induces changes in cell cycle behavior associated with inhibition of CDK1. A heterodimeric protein complex has been identified that links TRAF6 to IKK activation.3 Peptide mass fingerprinting analysis revealed that this complex is composed of the ubiquitin conjugating enzyme UBC13 and the UBC-like protein UBE2V1, also called UEV1A. TRAF6, a RING domain protein, functions together with UBC13/UEV1A to catalyze the synthesis of unique polyubiquitin chains linked through lysine-63 (K63) of ubiquitin. Blockade of this polyubiquitin chain synthesis, but not inhibition of the proteasome, prevents the activation of IKK by TRAF6. These results unveil a new regulatory function for ubiquitin, in which IKK is activated through the assembly of K63-linked polyubiquitin chains.
