Novel Multiubiquitin Chain Linkages Catalyzed by the Conjugating Enzymes E2EPF and RAD6 Are Recognized by 26 S Proteasome Subunit 5 (*)

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 ≤ 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.