title: Novel multiubiquitin chain linkages catalyzed by the conjugating enzymes E2EPF and RAD6 are recognized by 26 S proteasome subunit 5.
abstract: Targeting of substrates for degradation by the ATP, ubiquitin-dependent pathway requires formation of multiubiquitin chains in which the 8.6-kDa polypeptide is linked by isopeptide bonds between carboxyl termini and Lys-48 residues of successive monomers. Binding of Lys-48-linked chains by subunit 5 of the 26 S proteasome regulatory complex commits the attached target protein to degradation with concomitant release of free ubiquitin monomers following disassembly of the chains. Point mutants of ubiquitin (Lys-->Arg) were used to map the linkage specificity for ubiquitin-conjugating enzymes previously demonstrated to form novel multiubiquitin chains not attached through Lys-48. Recombinant human E2EPF catalyzed multiubiquitin chain formation exclusively through Lys-11 of ubiquitin while recombinant yeast RAD6 formed chains linked only through Lys-6. Multiubiquitin chains linked through Lys-6, Lys-11, or Lys-48 each bound to subunit 5 of partially purified human 26 S proteasome with comparable affinities. Since chains bearing different linkages are expected to pack into distinct structures, competition between Lys-11 and Lys-48 chains for binding to subunit 5 demonstrates that the latter possesses determinants for recognizing alternatively linked chains and precludes the existence of subunit 5 isoforms recognizing distinct structures. In addition, competition studies provided an estimate of Kd < or = 18 nM for the intrinsic binding of Lys-48-linked chains of linkage number n > 4. This result suggests that the principal mechanistic advantage of multiubiquitin chain formation is to enhance the affinity of the associated substrate for the 26 S complex relative to that of unconjugated target protein. Complementation studies with E1/E2-depleted rabbit reticulocyte extract demonstrated RAD6 supported isopeptide ligase-dependent degradation only through Lys-48-linked chains, while E2EPF retained the ability to target a model radiolabeled substrate through Lys-11-linked chains. Therefore, the linkage specificity exhibited by these E2 isozymes depends on their catalytic context with respect to isopeptide ligase.
authors.affiliation: Department of Biochemistry, Medical College of Wisconsin, Milwaukee 53226, USA.
authors.forename: O V
authors.initials: OV
authors.lastname: Baboshina
authors.forename: A L
authors.initials: AL
authors.lastname: Haas
mesh_terms: D064173:Anaphase-Promoting Complex-Cyclosome; D000818:Animals; D002384:Catalysis; D002417:Cattle; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D006801:Humans; D006868:Hydrolysis; D008025:Ligases / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D046988:Proteasome Endopeptidase Complex; D017510:Protein Folding; D013379:Substrate Specificity; D044763:Ubiquitin-Conjugating Enzymes; D043743:Ubiquitin-Protein Ligase Complexes; D044767:Ubiquitin-Protein Ligases; D014452:Ubiquitins / Q000235:genetics / Q000378:metabolism
chemical_list: D009097:Multienzyme Complexes; D014452:Ubiquitins; D044763:Ubiquitin-Conjugating Enzymes; D043743:Ubiquitin-Protein Ligase Complexes; D064173:Anaphase-Promoting Complex-Cyclosome; D044767:Ubiquitin-Protein Ligases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D008025:Ligases