The enzymes of the ubiquitylation pathway play a pivotal role in a number of cellular processes including regulated and targeted proteasomal degradation of substrate proteins. Three classes of enzymes are involved in the process of ubiquitylation; activating enzymes (E1s), conjugating enzymes (E2s) and protein ligases (E3s). UBE2K is a member of the E2 conjugating enzyme family and cloning of the human gene was first described by Kalchman et al. (1996). Human UBE2K shares 100% amino acid identity with bovine UBE2K and significant homology with yeast UBE2K, UBE2D2 and UBE2D1 (Kalchman et al., 1996). Interaction and selective binding of UBE2K to the N-terminus of huntingtin, the causal gene product in Huntington's disease has been demonstrated (Kalchman et al., 1996). In a yeast 2 hybrid screen binding of the RING finger protein RNF2 to UBE2K has also been determined (Christensen et al., 2007; Lee et al., 2001). UBE2K binds directly to the BRCA1 RING motif and is active with BRCA1-BARD1 in in vitro auto-ubiquitylation assays (Christensen et al., 2007). UBE2K directs the synthesis of Lys63- or Lys48-linked ubiquitin chains on BRCA1. UBE2K has been found to synthesize in vitro unanchored Lys(48)-linked poly-ubiquitin chains from mono- or poly-ubiquitin, E1, and ATP; thus, UBE2K has distinct binding sites for donor and acceptor (poly)Ub (Yao and Cohen, 2000). UBE2K was identified in a screen for novel SUMO targets, however attachment of SUMO to UBE2K in vitro severely impairs ubiquitin thioester and unanchored ubiquitin chain formation (Pichler et al., 2005). The ubiquitin-proteasome system malfunction in Alzheimer's disease (AD) has been attributed to neurotoxicity and proteasome inhibition by Abeta, which is mediated by an increase in the levels of UBE2K found in the brains of patients with AD. UBE2K's contribution to neurotoxicity is mediated by a ubiquitin B mutant (UBB+1), a potent inhibitor of proteasomes found in patients with AD (Song and Jung, 2004).
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