The C-terminal Extension of the β7 Subunit and Activator Complexes Stabilize Nascent 20 S Proteasomes and Promote Their Maturation

Abstract

The eukaryotic 20 S proteasome is formed by dimerization of two precursor complexes containing the maturation factor Ump1. β7/Pre4 is the only one of the 14 subunits forming the 20 S proteasome that is absent from these precursor complexes in Saccharomyces cerevisiae. Increased expression of Pre4 leads to a reduction in the level of precursor complex, indicating that Pre4 incorporation into these complexes is rate-limiting for their dimerization. When we purified these precursor complexes, we observed co-purification of Blm10, a large protein known to attach to the α ring surface of proteasomes. In contrast to single mutants lacking either Blm10 or the C-terminal extension of Pre4, a mutant lacking both grew extremely poorly, accumulated very high levels of precursor complexes, and was impaired in β subunit maturation. The effect of blm10Δ on proteasome biogenesis is modest, apparently because the 19 S regulatory particle is capable of substituting for Blm10, as long as precursor complex dimers are stabilized by the Pre4 C terminus. We found that a mutation (sen3/rpn2) affecting the Rpn2 subunit inhibits attachment of the 19 S activator to the 20 S particle or its precursors. Although the sen3 mutation alone had no apparent effect on precursor complex dimerization and active site maturation, the sen3 blm10 double mutant was impaired in these processes. Together these data demonstrate that Blm10 and the 19 S activator have a partially redundant function in stabilizing nascent 20 S proteasomes and in promoting their activation. The eukaryotic 20 S proteasome is formed by dimerization of two precursor complexes containing the maturation factor Ump1. β7/Pre4 is the only one of the 14 subunits forming the 20 S proteasome that is absent from these precursor complexes in Saccharomyces cerevisiae. Increased expression of Pre4 leads to a reduction in the level of precursor complex, indicating that Pre4 incorporation into these complexes is rate-limiting for their dimerization. When we purified these precursor complexes, we observed co-purification of Blm10, a large protein known to attach to the α ring surface of proteasomes. In contrast to single mutants lacking either Blm10 or the C-terminal extension of Pre4, a mutant lacking both grew extremely poorly, accumulated very high levels of precursor complexes, and was impaired in β subunit maturation. The effect of blm10Δ on proteasome biogenesis is modest, apparently because the 19 S regulatory particle is capable of substituting for Blm10, as long as precursor complex dimers are stabilized by the Pre4 C terminus. We found that a mutation (sen3/rpn2) affecting the Rpn2 subunit inhibits attachment of the 19 S activator to the 20 S particle or its precursors. Although the sen3 mutation alone had no apparent effect on precursor complex dimerization and active site maturation, the sen3 blm10 double mutant was impaired in these processes. Together these data demonstrate that Blm10 and the 19 S activator have a partially redundant function in stabilizing nascent 20 S proteasomes and in promoting their activation. The multicatalytic protease, termed “26 S proteasome,” provides the main and essential proteolytic activity in the cytoplasm and the nucleus of a eukaryotic cell (1Baumeister W. Walz J. Zühl F. Seemüller E. Cell. 1998; 92: 367-380Abstract Full Text Full Text PDF PubMed Scopus (1310) Google Scholar). The main function of this protease is to degrade polyubiquitylated substrates, which include abnormal as well as regulatory proteins. Because of these properties, the proteasome has become an interesting drug target with potential in the treatment of cancer (2Adams J. Nat. Rev. Cancer. 2004; 4: 349-360Crossref PubMed Scopus (1076) Google Scholar). The 26 S proteasome is composed of the catalytic core particle (CP), 4The abbreviations used are: CP, core particle; RP, regulatory particle; CTE, C-terminal extension; HA, hemagglutinin; FH-Ump1, FLAG-His6-tagged Ump1. termed the “20 S proteasome,” and two 19 S regulatory particles (RPs), also known as PA700 activator complexes (3Hanna J. Finley D. FEBS Lett. 2007; 581: 2854-2861Crossref PubMed Scopus (139) Google Scholar). The eukaryotic 20 S proteasome is assembled from 14 distinct subunits, 7 of the α type and 7 of the β type. The overall α7β7β7α7 structure of this particle is characterized by the presence of two identical halves, each comprising a ring of seven α subunits and a ring of seven β subunits (4Groll M. Ditzel L. Löwe J. Stock D. Bochtler M. Bartunik H.D. Huber R. Nature. 1997; 386: 463-471Crossref PubMed Scopus (1951) Google Scholar, 5Unno M. Mizushima T. Morimoto Y. Tomisugi Y. Tanaka K. Yasuoka N. Tsukihara T. Structure. 2002; 10: 609-618Abstract Full Text Full Text PDF PubMed Scopus (446) Google Scholar). The two inner β rings contact each other and form the catalytic chamber of the CP. Five of the β subunits are synthesized as precursors with N-terminal propeptides (6Heinemeyer W. Ramos P.C. Dohmen R.J. Cell. Mol. Life Sci. 2004; 61: 1562-1578Crossref PubMed Google Scholar). Three of these subunits, β1, β2, and β5, provide the postacidic, the tryptic, and the chymotryptic activities, respectively (7Arendt C.S. Hochstrasser M. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 7156-7161Crossref PubMed Scopus (249) Google Scholar, 8Heinemeyer W. Fischer M. Krimmer T. Stachon U. Wolf D.H. J. Biol. Chem. 1997; 272: 25200-25209Abstract Full Text Full Text PDF PubMed Scopus (409) Google Scholar). The two outer α rings contact the β rings on one side, and the base of the 19 S RP or alternative activator complexes on the other side. The 19 S RP is composed of two subcomplexes, the base and the lid, that are linked by the Rpn10 subunit (9Glickman M.H. Rubin D.M. Coux O. Wefes I. Pfeifer G. Cjeka Z. Baumeister W. Fried V.A. Finley D. Cell. 1998; 94: 615-623Abstract Full Text Full Text PDF PubMed Scopus (774) Google Scholar). The base is composed of six ATPase subunits (Rpt1–Rpt6) and three non-ATPase subunits (Rpn1, Rpn2, and Rpn13) (3Hanna J. Finley D. FEBS Lett. 2007; 581: 2854-2861Crossref PubMed Scopus (139) Google Scholar). The 19 S RP has multiple roles in proteasome function. It recognizes ubiquitylated proteins either directly or indirectly and mediates their unfolding and translocation into the CP. In addition, the 19 S RP serves as an interaction platform for numerous other proteins, including ubiquitin ligases and deubiquitylating enzymes (3Hanna J. Finley D. FEBS Lett. 2007; 581: 2854-2861Crossref PubMed Scopus (139) Google Scholar, 10Demartino G.N. Gillette T.G. Cell. 2007; 129: 659-662Abstract Full Text Full Text PDF PubMed Scopus (189) Google Scholar). Aside from the 19 S RP, several activator complexes have been described that can alternatively attach to the 20 S proteasome and activate its proteolytic function. However, only the 19 S RP appears to promote the degradation of ubiquitylated proteins. In Saccharomyces cerevisiae, the HEAT repeat protein Blm10, a homologue of mammalian PA200 (11Ustrell V. Hoffman L. Pratt G. Rechsteiner M. EMBO J. 2002; 21: 3516-3525Crossref PubMed Scopus (272) Google Scholar), has been reported to attach to 20 S CPs and to activate its peptide cleavage activity in vitro (12Schmidt M. Haas W. Crosas B. Santamaria P.G. Gygi S.P. Walz T. Finley D. Nat. Struct. Mol. Biol. 2005; 12: 294-303Crossref PubMed Scopus (151) Google Scholar, 13Iwanczyk J. Sadre-Bazzaz K. Ferrell K. Kondrashkina E. Formosa T. Hill C.P. Ortega J. J. Mol. Biol. 2006; 363: 648-659Crossref PubMed Scopus (46) Google Scholar). Blm10 (called Blm3 in some publications), however, has also been detected in association with proteasome precursor complexes (14Fehlker M. Wendler P. Lehmann A. Enenkel C. EMBO Rep. 2003; 4: 959-963Crossref PubMed Scopus (99) Google Scholar, 15Li X. Kusmierczyk A.R. Wong P. Emili A. Hochstrasser M. EMBO J. 2007; 26: 2339-2349Crossref PubMed Scopus (118) Google Scholar). The physiological function of Blm10 has remained unclear. 20 S CPs are assembled from “half-proteasome precursor complexes” containing unprocessed β subunits and maturation factor Ump1 (underpinning maturation of proteasome) (6Heinemeyer W. Ramos P.C. Dohmen R.J. Cell. Mol. Life Sci. 2004; 61: 1562-1578Crossref PubMed Google Scholar, 16Krüger E. Kloetzel P.M. Enenkel C. Biochimie (Paris). 2001; 83: 289-293Crossref PubMed Scopus (64) Google Scholar). Following dimerization of such complexes, autocatalytic processing and thereby activation of β1, β2, and β5 as well as degradation of Ump1 take place (17Chen P. Hochstrasser M. Cell. 1996; 86: 961-972Abstract Full Text Full Text PDF PubMed Scopus (326) Google Scholar, 18Ramos P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar). We have reported recently that a C-terminal extension (CTE) of S. cerevisiae β7/Pre4 serves two functions in 20 S CP biogenesis (19Ramos P.C. Marques A.J. London M.K. Dohmen R.J. J. Biol. Chem. 2004; 279: 14323-14330Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). This extension emanates from one half of the CP to the other, where it intercalates between subunits β1/Pre3 and β2/Pup1, thereby promoting formation and of nascent the Pre4 CTE, in addition, contact in β1/Pre3 to a that is to promote β1/Pre3 processing and activity (4Groll M. Ditzel L. Löwe J. Stock D. Bochtler M. Bartunik H.D. Huber R. Nature. 1997; 386: 463-471Crossref PubMed Scopus (1951) Google Scholar, P.C. Marques A.J. London M.K. Dohmen R.J. J. Biol. Chem. 2004; 279: 14323-14330Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). We that purified precursor complexes that β7/Pre4 incorporation and is rate-limiting for its dimerization. of the alone has effect on 20 S CP it of precursor complexes, and β subunit processing either with a of the of Pre4 or with a mutation in the The of these that Blm10, the 19 S RP, and the Pre4 have redundant functions in 20 S CP formation and maturation. and and with as described (19Ramos P.C. Marques A.J. London M.K. Dohmen R.J. J. Biol. Chem. 2004; 279: 14323-14330Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). are of (19Ramos P.C. Marques A.J. London M.K. Dohmen R.J. J. Biol. Chem. 2004; 279: 14323-14330Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). and have been described P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar). Pre4 protein lacking its C-terminal 19 was by was used to a of that FLAG-His6-tagged Ump1 and which Blm10 from and a as described P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar). used to and and by the from the from the and it into or by was to to blm10Δ double for several of this of or from the a of was The was used as an of by and cerevisiae in or to of and on a to an protein as well as and as described P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar). The of a was of which by and in to of and with in was and the protein in the was the protein the chymotryptic of each to a of in containing of The of the peptide with of the was for of protein from each and of the as described R. K. Dohmen R.J. EMBO J. 2004; PubMed Scopus Google Scholar). The used or of the and of in of and with in and in and and with the for was for The was with a of for The was three for each with of and the was for in containing peptide The was on a and with as described V. R. I. N. W. D. PubMed Scopus Google Scholar). from the and the proteins and The used for protein and of have the Pre4 and maturation factor Ump1 in 20 S CP and maturation P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar, P.C. Marques A.J. London M.K. Dohmen R.J. J. Biol. Chem. 2004; 279: 14323-14330Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). the of these we purified the proteasome precursor complex from S. cerevisiae Because this complex is of in we of that the mutation in a of its Ump1 is to the precursor it is absent from 20 and 26 S we to this maturation factor to a for Because Ump1 the dimerization of these precursor complexes P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar), this in addition, has the that of Ump1 to an dimerization. C-terminal of Ump1 are the precursor complex P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar), we a to its We found this in with the mutation to of the precursor containing purified by a on a containing of the purified that it precursor complexes and no 20 or 26 S particles of the purified precursor complex by and by peptide This the presence of of the 14 CP the subunit was In addition, we detected Ump1 and that used Ump1 its C with protein for complexes containing complexes, however, and and (14Fehlker M. Wendler P. Lehmann A. Enenkel C. EMBO Rep. 2003; 4: 959-963Crossref PubMed Scopus (99) Google Scholar, A. K. B. Kloetzel P.M. Enenkel C. J. Mol. Biol. 2002; PubMed Scopus Google Scholar). the between these are to the C-terminal protein which in an assembled complex with a peptide the C of Ump1 was to to in the of the precursor complex, indicating that the C of the protein into the of the complex P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar). β7/Pre4 a in 20 S CP that β7/Pre4 is the only subunit in 20 S proteasomes that is absent from purified precursor complexes that it is the subunit to into such complexes, their dimerization. with this in contrast to no of detected in of a that the precursor complex and with from an identical and with an from the of Pre4 in and we also Pre4 in these The that Pre4 is a rate-limiting subunit its is into precursor complexes and their dimerization. the mutant accumulated levels of the precursor complex, we the Pre4, which is impaired in 20 S CP in these This was the We that incorporation of Pre4 and 20 S CP formation on each In contrast to was observed for Pre4, we detected of Blm10 in containing precursor complexes in the mutant in with the type data are with the of precursor complexes and that Blm10 to precursor complexes lacking of β7/Pre4 of apparent of Pre4 subunit and its from the precursor complex that its incorporation or is by precursor complex dimerization and that Pre4 of complexes in a with in a this with the a β7/Pre4 of the precursor complex unprocessed of the of unprocessed of in this and of or in had no effect on the of data the that the observed rate-limiting for dimerization of precursor complexes is a function of for 20 S CP and in the presence of Blm10, a large protein reported to function as a regulatory complex that to α rings of 20 S CPs in proteasome precursor to its has an on proteasome The blm10Δ mutation had a very effect on this with of unprocessed by a data apparent on processing or activity or In levels of proteasome precursor detected in the single or The blm10Δ double in a and was that this mutant also activity and was impaired in processing and which in proteasome biogenesis are we the of complexes in this mutant with of type and the single mutants In the double we observed a of precursor complexes, in the single In contrast to the the double mutant a in 20 S CP maturation. was detected in containing the 20 and 26 S Together these that Blm10 has a in proteasome and maturation. In the of Blm10, these to the nascent by of the from Pre4 this of Blm10 the of the Pre4 CTE, precursor has no apparent effect on maturation. of the CTE, however, in a of processing in a lacking these that both Blm10 and the Pre4 nascent 20 S thereby promoting their maturation. The sen3 of the 19 S RP to the 20 S protein is a subunit of the base of the 19 S RP (9Glickman M.H. Rubin D.M. Coux O. Wefes I. Pfeifer G. Cjeka Z. Baumeister W. Fried V.A. Finley D. Cell. 1998; 94: 615-623Abstract Full Text Full Text PDF PubMed Scopus (774) Google Scholar). We the effect of the mutation S. D. Hochstrasser M. Mol. Cell. Biol. PubMed Scopus Google on complexes by We observed a of complexes in by Although 20 S CP with or 19 S RP, in of only in of The the 20 S 20 S proteasomes with one or two 19 S are R.J. London M.K. C. Ramos P.C. Mol. Biol. 2005; Google Scholar). This that inhibits of the 19 S complex with the 20 S CP. Rpn2 and Blm10 for in 20 S of the mutant described to the of the 19 S RP a in the and maturation of the 20 S CP. to the blm10Δ mutant the mutant however, an in precursors levels a in the processing of We with blm10Δ in such this mutant Blm10 and have 19 S This double mutant grew the single mutant and a of precursor complexes containing unprocessed and We that the 19 S RP can for Blm10 in stabilizing nascent 20 S proteasomes and promoting their maturation. this we of the 19 S RP detected in association with proteasome precursor an ATPase subunit of the 19 S RP, with from of lacking Blm10 was detected in the from containing Blm10 used We that the 19 S RP to precursor complexes in the of In the we have the that dimerization of proteasome precursor complexes and the maturation that with it in S. cerevisiae. β7/Pre4 of found that purified precursor complexes seven α subunits, six of the seven β subunits one and data are with which was was in the X. Kusmierczyk A.R. Wong P. Emili A. Hochstrasser M. EMBO J. 2007; 26: 2339-2349Crossref PubMed Scopus (118) Google Scholar). also that Pre4 incorporation is a rate-limiting in dimerization of precursor complexes, the that of Pre4 the of several proteasome data provide for Pre4 incorporation a rate-limiting in proteasome by that of Pre4 in in a of precursor complex dimerization of Pre4 in that β7/Pre4 is as a subunit and that Pre4 in a reduction of the level of proteasome precursor complexes this subunit an for regulatory to of other subunits, the of the that are by activator G. R. V. I. FEBS Lett. PubMed Scopus Google Scholar, R.J. I. Marques A.J. 2007; PubMed Scopus Google Scholar). is a factor in an that of proteasome in to or of the proteasome Y. Varshavsky A. Proc. Natl. Acad. Sci. U. S. A. 2001; PubMed Scopus Google Scholar, M.K. Ramos P.C. Dohmen R.J. FEBS Lett. 2004; PubMed Scopus Google Scholar). of expression of with of other proteasome subunit from their also a distinct of to of a protein the Saccharomyces A.R. D. J. S. Nat. PubMed Scopus Google Scholar). It is to such expression in an of precursor complexes and thereby in levels of of proteasome levels by such a an expression of proteasome subunit In this precursor complexes lacking Pre4 a form of complexes to by incorporation of Pre4 in to Blm10 to and 20 S with other (14Fehlker M. Wendler P. Lehmann A. Enenkel C. EMBO Rep. 2003; 4: 959-963Crossref PubMed Scopus (99) Google Scholar, 15Li X. Kusmierczyk A.R. Wong P. Emili A. Hochstrasser M. EMBO J. 2007; 26: 2339-2349Crossref PubMed Scopus (118) Google Scholar), we detected Blm10 in purified precursor of the Blm10 had only a effect on proteasome maturation. This was by the of to in and blm10Δ Although and or no in these the of in blm10Δ observed we blm10Δ with the of a of Pre4 by levels of proteasome indicating that it has a in stabilizing the nascent 20 S proteasome precursor complex dimerization (19Ramos P.C. Marques A.J. London M.K. Dohmen R.J. J. Biol. Chem. 2004; 279: 14323-14330Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar). of the presence of the Pre4 CTE, 20 and 26 S proteasome complexes formed in its It is that we of in containing complexes in the mutant This β subunits have in the nascent the 20 S CP is the Pre4 Although the of the of the Pre4 or of Blm10 by are the double mutant lacking both grew the accumulated of proteasome precursor complexes, and impaired processing of in 20 and 26 S This interaction that both the Pre4 and Blm10 to a and maturation of nascent CP. as Blm10 is this appears to in with a in which Blm10 was to proteasome maturation. This was that a of β subunit processing and of Ump1 degradation in blm10Δ (14Fehlker M. Wendler P. Lehmann A. Enenkel C. EMBO Rep. 2003; 4: 959-963Crossref PubMed Scopus (99) Google Scholar). in Blm10 as a factor promoting proteasome maturation. of in the effect of blm10Δ alone on proteasome and maturation that an alternative activator complex take this function in the of We the 19 S RP this we of indicating that a mutation affecting its Rpn2 subunit in a of 19 S RP attachment to the 20 S CP Although the mutation impaired formation of 26 S it had no effect on the level of precursor complex or on β subunit processing When we this mutation with which by also on proteasome maturation, we found the double mutant to impaired in 20 S CP It is that in this 20 S subunits and Ump1 are these data that activator complexes are to nascent 20 S CPs and to promote their activation by β subunit maturation. 19 S RP or Blm10 apparently can alternatively this function. The effect of both on the maturation of 20 S however, was that observed in the blm10Δ double is that nascent 20 S CPs that are stabilized by the Pre4 have the to is that proteins or complexes other the 19 S RP or Blm10 as alternative of nascent proteasomes. protein is which to 20 S CP and its to the 19 S RP J. A. Crosas B. M. Walz T. Finley D. Mol. Cell. 2002; 10: Full Text Full Text PDF PubMed Scopus Google Scholar). of the a in with blm10Δ (12Schmidt M. Haas W. Crosas B. Santamaria P.G. Gygi S.P. Walz T. Finley D. Nat. Struct. Mol. Biol. 2005; 12: 294-303Crossref PubMed Scopus (151) Google Scholar). for in 20 S CP and maturation by that to the α rings that to a of CP either directly or indirectly by promoting β subunit maturation. The of such which are apparent in the α have been well for the attachment of the S to the S. cerevisiae 20 S CP L. Y. Hill C.P. Nature. PubMed Scopus Google Scholar). of the S however, appears to in S. cerevisiae. in which the Blm10 or 19 S RP promote β subunit maturation, is in with the that the observed in the sen3 blm10Δ double mutant include of precursor complex and maturation of 20 S particles the that these activator complexes to the 20 S CP that are to the β subunits, it is to that promote autocatalytic processing of β subunits as well as cleavage of This provides a for two distinct one side, are that demonstrate an activation of in vitro activity of the 20 S CP by Blm10 (12Schmidt M. Haas W. Crosas B. Santamaria P.G. Gygi S.P. Walz T. Finley D. Nat. Struct. Mol. Biol. 2005; 12: 294-303Crossref PubMed Scopus (151) Google Scholar, 13Iwanczyk J. Sadre-Bazzaz K. Ferrell K. Kondrashkina E. Formosa T. Hill C.P. Ortega J. J. Mol. Biol. 2006; 363: 648-659Crossref PubMed Scopus (46) Google Scholar). the other side, Blm10 is found in association with precursor complexes (14Fehlker M. Wendler P. Lehmann A. Enenkel C. EMBO Rep. 2003; 4: 959-963Crossref PubMed Scopus (99) Google Scholar, 15Li X. Kusmierczyk A.R. Wong P. Emili A. Hochstrasser M. EMBO J. 2007; 26: 2339-2349Crossref PubMed Scopus (118) Google and their dimerization and maturation of active both on and mammalian 20 S proteasomes and their precursor complexes have that are precursor complex dimerization and proteasome maturation D. E. EMBO J. 1997; PubMed Scopus Google Scholar, S. M. K. M. Baumeister W. Structure. 2006; Full Text Full Text PDF PubMed Scopus Google Scholar, S. L. R. Kloetzel P.M. E. J. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). It is to that multiple including of the Pre4 and of β subunit propeptides into the other half of the proteasome X. Kusmierczyk A.R. Wong P. Emili A. Hochstrasser M. EMBO J. 2007; 26: 2339-2349Crossref PubMed Scopus (118) Google Scholar, P.C. Marques A.J. London M.K. Dohmen R.J. J. Biol. Chem. 2004; 279: 14323-14330Abstract Full Text Full Text PDF PubMed Scopus (54) Google Scholar), and degradation of Ump1 in the nascent CP P.C. Höckendorff J. Johnson E.S. Varshavsky A. Dohmen R.J. Cell. 1998; 92: 489-499Abstract Full Text Full Text PDF PubMed Scopus (262) Google Scholar), β subunit maturation (17Chen P. Hochstrasser M. Cell. 1996; 86: 961-972Abstract Full Text Full Text PDF PubMed Scopus (326) Google Scholar), and the attachment of activator complexes to the α rings C. Z. Hill C.P. Rechsteiner M. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar), to such the of eukaryotic proteasomes. in which Blm10 functions in promoting 20 S CP maturation is with the that Blm10, to the 19 S also as a activator of 20 S however, provide that Blm10 in proteasome to functions as a activator of 20 S CP is of that Blm10 is the 19 S RP in to precursor complexes and in promoting their dimerization and 20 S particle maturation. Blm10 was found in high that in in purified precursor complexes 19 S RP subunits, in only detected such complexes from a lacking Blm10 by Blm10 is capable of promoting precursor complex dimerization and 20 S CP maturation the is by a of the Pre4 C-terminal extension The effect of the blm10 and that the 19 S RP for Blm10 in the proteasome the precursor complex interaction is by the of the Pre4 We a in which proteasome activator complexes are for dimerization of precursor complexes and 20 S particle β subunit maturation. data that is a of in which Blm10 functions in proteasome 19 S RP functions in association with 20 S particles or in 20 S processes. The have been to include functions in and the of Gillette T.G. 2007; PubMed Scopus Google Scholar). We for protein for Hochstrasser for the for and and of for peptide