Ubox

Modified RING finger domain
Ubox
SMART accession number:SM00504
Description: Modified RING finger domain, without the full complement of Zn2+-binding ligands. Probable involvement in E2-dependent ubiquitination.
Interpro abstract (IPR003613):

Quality control of intracellular proteins is essential for cellular homeostasis. Molecular chaperones recognise and contribute to the refolding of misfolded or unfolded proteins, whereas the ubiquitin-proteasome system mediates the degradation of such abnormal proteins. Ubiquitin-protein ligases (E3s) determine the substrate specificity for ubiquitylation and have been classified into HECT and RING-finger families. More recently, however, U-box proteins, which contain a domain (the U box) of about 70 amino acids that is conserved from yeast to humans, have been identified as a new type of E3 [(PUBMED:12944364)].

Members of the U-box family of proteins constitute a class of ubiquitin-protein ligases (E3s) distinct from the HECT-type and RING finger-containing E3 families [(PUBMED:12944364)]. Using yeast two-hybrid technology, all mammalian U-box proteins have been reported to interact with molecular chaperones or co-chaperones, including Hsp90, Hsp70, DnaJc7, EKN1, CRN, and VCP. This suggests that the function of U box-type E3s is to mediate the degradation of unfolded or misfolded proteins in conjunction with molecular chaperones as receptors that recognise such abnormal proteins [(PUBMED:15115282), (PUBMED:15189447)].

Unlike the RING finger domain, IPR001841, that is stabilised by Zn2+ ions coordinated by the cysteines and a histidine, the U-box scaffold is probably stabilised by a system of salt-bridges and hydrogen bonds. The charged and polar residues that participate in this network of bonds are more strongly conserved in the U-box proteins than in classic RING fingers, which supports their role in maintaining the stability of the U box. Thus, the U box appears to have evolved from a RING finger domain by appropriation of a new set of residues required to stabilise its structure, concomitant with the loss of the original, metal-chelating residues [(PUBMED:10704423)].

GO process:protein ubiquitination (GO:0016567)
GO component:ubiquitin ligase complex (GO:0000151)
GO function:ubiquitin-protein ligase activity (GO:0004842)
Family alignment:
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There are 1335 Ubox domains in 1330 proteins in SMART's nrdb database.

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