Structural Basis of E2–25K/UBB+1 Interaction Leading to Proteasome Inhibition and Neurotoxicity

Abstract

E2-25K/Hip2 is an unusual ubiquitin-conjugating enzyme that interacts with the frameshift mutant of ubiquitin B (UBB ؉1 ) and has been identified as a crucial factor regulating amyloid-␤ neurotoxicity. To study the structural basis of the neurotoxicity mediated by the E2-25K-UBB ؉1 interaction, we determined the three-dimensional structures of UBB ؉1 , E2-25K and the ), and polyglutamine-expanded huntingtin (2). UBB ϩ1 was first identified as a frameshift mutant of the ubiquitin (Ub) B protein in the brains of neurodegenerative disease patients (3) and is composed of a Ub moiety (75 residues) with a 19-residue C-terminal extension. Neither A␤ nor UBB ϩ1 is found in young patients not suffering from dementia, but they are observed in older Alzheimer disease patients (4). The genes from which UBB ϩ1 mRNAs are transcribed contain several GAGAG motifs, and dinucleotide deletions (⌬GA) from within the GAGAG motif result in an abnormal C-terminal sequence. Normally these aberrant proteins are removed by the Ub-proteasome system (UPS), which executes the proteolytic degradation of aberrant proteins via a Ub-tagging mechanism (3, 5). E2-25K/ubiquitin, and E2-25K/UBBWithin the UPS, Ub tagging of target molecules entails enzymatic reactions catalyzed by the E1 (Ub-activating), E2 (Ubconjugating), and E3 (Ub-ligating) enzymes. Once E3 tags a target molecule with mono-or polyUb, the tagged molecule is recognized by the 26 S proteasome and degraded (6). In the healthy brain both ␤-amyloid precursor protein and UBB ϩ1 molecules are targets for the UPS and are degraded by the 26 S proteasome (7,8). In the brains of Alzheimer patients, however, both UBB ϩ1 and Ub are present within aggregation plaques also containing ␤-amyloid precursor protein, which is indicative of UPS dysfunction (9, 10). When at normal basal levels, UBB ϩ1 can be removed by the UPS. But when its expression is up-regulated, UBB ϩ1 inhibits the 26 S proteasome in a dose-dependent manner, resulting in the accumulation of aberrant proteins (11). The aberrant C terminus of UBB ϩ1 prevents its activation and, therefore, subsequent ligation to substrates due to , the frame shift mutant of ubiquitin B; UPS, ubiquitin-proteasome system; HSQC, heteronuclear single quantum correlation; TROSY, transverse relaxation optimized spectroscopy; DsRed, Discosoma sp. red fluorescent protein.

Relationship between amyloid-beta and the ubiquitin–proteasome system in Alzheimer’s disease

“…In addition, the interaction between E2-25K/Hip-2 and UBB(z1) may be critical for the synthesis and accumulation of UBB(z1)-anchored polyubiquitin, which results in proteasomal inhibition and death of the neurons. 21 Single nucleotide polymorphisms in the UBQLN1 gene have been linked to late-onset AD. Its protein product, ubiquilin-1, functions as a molecular chaperone for APP and leads to Abeta accumulation.…”

“…6 E2-25K is an unusual member of the E2 conjugating enzyme family in that it is competent to catalyze Ub chain extension independent of E3 ligases. 21 It functions upstream of apoptosis signal-regulating kinase 1 (ASK1) and c-Jun N-terminal kinase (JNK) in Abeta42 toxicity. 6 It contains a UBA domain that is unique to human E2 conjugating enzymes.…”

Ubiquitin proteasome system networks in the neurological disorders

“…A number of studies have shown that neurotoxicity of Aβs is critical to pathogenesis of AD. A current report found that Aβs are also regulated by UPS components, such as E2-25/Hip2 and TRIP22 (Ko et al 2010). HRD1 regulates levels of APP in the AD brains in an Ubdependent manners (Kaneko et al 2010).…”

C-terminal acidic domain of ubiquitin-conjugating enzymes: A multi-functional conserved intrinsically disordered domain in family 3 of E2 enzymes

“…The sequence of the 3D structures of UBA domains of Saccharomyces cerevisiae Ubc1 (pdb entry 1TTE (Merkley and Shaw, 2004)) and human Ube2 K (pdb entry 3K9P (Ko et al, 2010)) were used as query in Blast-p to retrieve other UBA domains of E2 family 1, sharing different degree of sequence similarity with the queries. Moreover, other UBA domains belonging to proteins different from E2 enzymes were retrieved by a Blast-p search in PDB.…”