ubc-9

SUMO-Conjugating Enzyme UBC9 (UBC9) belongs to the ubiquitin-conjugating enzyme family. UBC9 is homologous to ubiquitin-conjugating enzymes (E2s). However, instead of conjugating ubiquitin, UBC9 conjugates a ubiquitin homologue, Small Ubiquitin-Like Modifier 1 (SUMO-1). The conjugation of ubiquitin requires the activities of ubiquitin-activating (E1) and conjugating (E2) enzymes. It is suggested that UBC9 might play a role in DNA repair and perhaps even in aging.

UBE2I 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.