ubc-2

Ubiquitin-conjugating enzyme E2 A (also known as HHR6A or UBE2A), encoded by human DNA repair genes HHR6A, belongs to the ubiquitin-conjugating enzymes (E2 enzymes) family and is likely to be involved in postreplication repair and induced mutagenesis. UBE2A is described as a CDK2 substrate. It is the human homologue of the product of the Saccharomyces cerevisiae RAD6 / UBC2 gene, a member of the family of ubiquitin-conjugating enzymes. In vivo, HHR6A phosphorylation peaks during the G2/M phase of cell cycle transition, with a concomitant increase in histone H2B ubiquitylation. Mutation of Ser120 to threonine or alanine abolished UBE2A activity, while mutation to aspartate to mimic phosphorylated serine increased UBE2A activity 3-fold. A mutation of UBE2A is considered as the cause of a novel X-linked mental retardation (XLMR) syndrome that affects three males in a two-generation family.

UBE2H is a member of the ubiquitin-conjugating E2 family whose members perform the second step in the ubiquitination reaction. Initially identified as the main process for protein degradation, ubiquitination is believed nowadays to be crucial for a wider range of cellular processes. The outcome of the ubiquitin-conjugation reaction, and thereby the fate of the substrate, is heavily dependent on the number of ubiquitin molecules attached and how these ubiquitin molecules are inter-connected. To deal with this complexity and to allow adequate ubiquitination in time and space, a highly sophisticated conjugation machinery has been developed. In a sequential manner, ubiquitin becomes activated by a ubiquitin-activating enzyme (E1), which then transfers the ubiquitin to a group of ubiquitin-conjugating enzymes (E2s). Next, ubiquitin-loaded E2s are interacting with ubiquitin-protein ligases (E3s) and ubiquitin is conjugated to substrates on recruitment by the E3. These three key enzymes are operating in a hierarchical system, wherein two E1s and 35 E2s have been found and hundreds of E3s have been identified in humans.