title: Prefoldin subunits are protected from ubiquitin-proteasome system-mediated degradation by forming complex with other constituent subunits.
abstract: The molecular chaperone prefoldin (PFD) is a complex comprised of six different subunits, PFD1-PFD6, and delivers newly synthesized unfolded proteins to cytosolic chaperonin TRiC/CCT to facilitate the folding of proteins. PFD subunits also have functions different from the function of the PFD complex. We previously identified MM-1α/PFD5 as a novel c-Myc-binding protein and found that MM-1α suppresses transformation activity of c-Myc. However, it remains unclear how cells regulate protein levels of individual subunits and what mechanisms alter the ratio of their activities between subunits and their complex. In this study, we found that knockdown of one subunit decreased protein levels of other subunits and that transfection of five subunits other than MM-1α into cells increased the level of endogenous MM-1α. We also found that treatment of cells with MG132, a proteasome inhibitor, increased the level of transfected/overexpressed MM-1α but not that of endogenous MM-1α, indicating that overexpressed MM-1α, but not endogenous MM-1α, was degraded by the ubiquitin proteasome system (UPS). Experiments using other PFD subunits showed that the UPS degraded a monomer of PFD subunits, though extents of degradation varied among subunits. Furthermore, the level of one subunit was increased after co-transfection with the respective subunit, indicating that there are specific combinations between subunits to be stabilized. These results suggest mutual regulation of protein levels among PFD subunits and show how individual subunits form the PFD complex without degradation.
authors.affiliation: Graduate School of Pharmaceutical Sciences, Hokkaido University, Kita-ku, Sapporo, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Miyazawa
authors.forename: Erika
authors.initials: E
authors.lastname: Tashiro
authors.forename: Hirotake
authors.initials: H
authors.lastname: Kitaura
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Maita
authors.forename: Hiroo
authors.initials: H
authors.lastname: Suto
authors.forename: Sanae M M
authors.initials: SM
authors.lastname: Iguchi-Ariga
authors.forename: Hiroyoshi
authors.initials: H
authors.lastname: Ariga
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D057809:HEK293 Cells; D006367:HeLa Cells; D056945:Hep G2 Cells; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D051379:Mice; D046912:Multiprotein Complexes / Q000235:genetics / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism*; D061988:Proteasome Inhibitors; D017510:Protein Folding; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism*; D025801:Ubiquitin / Q000235:genetics / Q000378:metabolism
chemical_list: D002352:Carrier Proteins; D015853:Cysteine Proteinase Inhibitors; D007976:Leupeptins; C513593:MM-1 protein, mouse; C489427:MYC protein, human; D046912:Multiprotein Complexes; C489428:Myc protein, mouse; C115636:PFDN5 protein, human; D061988:Proteasome Inhibitors; D016271:Proto-Oncogene Proteins c-myc; D012097:Repressor Proteins; D025801:Ubiquitin; D046988:Proteasome Endopeptidase Complex; C072553:benzyloxycarbonylleucyl-leucyl-leucine aldehyde