From Proteopediaproteopedia link
Solution Structure of the Internal UBA Domain of HHR23A
[RD23A_HUMAN] Multiubiquitin chain receptor involved in modulation of proteasomal degradation. Binds to 'Lys-48'-linked polyubiquitin chains in a length-dependent manner and with a lower affinity to 'Lys-63'-linked polyubiquitin chains. Proposed to be capable to bind simultaneously to the 26S proteasome and to polyubiquitinated substrates and to deliver ubiquitinated proteins to the proteasome.     Involved in nucleotide excision repair and is thought to be functional equivalent for RAD23B in global genome nucleotide excision repair (GG-NER) by association with XPC. In vitro, the XPC:RAD23A dimer has NER activity. Can stabilize XPC.     Involved in vpr-dependent replication of HIV-1 in non-proliferating cells and primary macrophages. Required for the association of HIV-1 vpr with the host proteasome.    
Publication Abstract from PubMed
UBA domains are a commonly occurring sequence motif of approximately 45 amino acid residues that are found in diverse proteins involved in the ubiquitin/proteasome pathway, DNA excision-repair, and cell signaling via protein kinases. The human homologue of yeast Rad23A (HHR23A) is one example of a nucleotide excision-repair protein that contains both an internal and a C-terminal UBA domain. The solution structure of HHR23A UBA(2) showed that the domain forms a compact three-helix bundle. We report the structure of the internal UBA(1) domain of HHR23A. Comparison of the structures of UBA(1) and UBA(2) reveals that both form very similar folds and have a conserved large hydrophobic surface patch. The structural similarity between UBA(1) and UBA(2), in spite of their low level of sequence conservation, leads us to conclude that the structural variability of UBA domains in general is likely to be rather small. On the basis of the structural similarities as well as analysis of sequence conservation, we predict that this hydrophobic surface patch is a common protein-interacting surface present in diverse UBA domains. Furthermore, accumulating evidence that ubiquitin binds to UBA domains leads us to the prediction that the hydrophobic surface patch of UBA domains interacts with the hydrophobic surface on the five-stranded beta-sheet of ubiquitin. Detailed comparison of the structures of the two UBA domains, combined with previous mutagenesis studies, indicates that the binding site of HIV-1 Vpr on UBA(2) does not completely overlap the ubiquitin binding site.
Solution structures of UBA domains reveal a conserved hydrophobic surface for protein-protein interactions.,Mueller TD, Feigon J J Mol Biol. 2002 Jun 21;319(5):1243-55. PMID:12079361
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.