title: Properties of the hybrid form of the 26S proteasome containing both 19S and PA28 complexes.
abstract: PA28 is a gamma-interferon-induced complex that associates with the 20S proteasome and stimulates breakdown of small peptides. Recent immunoprecipitation studies indicate that, in vivo, PA28 also exists in larger complexes that also contain the 19S particle, which is required for ATP-ubiquitin-dependent degradation of proteins. However, because of its lability, the structure and properties of this larger complex remain unclear. Here, we demonstrate that, in vitro, PA28 can associate with 'singly capped' 26S (i.e. 19S-20S) proteasomes. Electron microscopy of the resulting structures revealed one PA28 ring at one end of the 20S particle and a 19S complex at the other. These hybrid complexes show enhanced hydrolysis of small peptides, but no significant increase in rates of protein breakdown. Nevertheless, during breakdown of proteins, the complexes containing PA28alphabeta or PA28alpha generated a pattern of peptides different from those generated by 26S proteasomes, without altering mean product length. Presumably, this change in peptides produced accounts for the capacity of PA28 to enhance antigen presentation.
authors.affiliation: Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA.
authors.forename: Paolo
authors.initials: P
authors.lastname: Cascio
authors.forename: Matthew
authors.initials: M
authors.lastname: Call
authors.forename: Benjamin M
authors.initials: BM
authors.lastname: Petre
authors.forename: Thomas
authors.initials: T
authors.lastname: Walz
authors.forename: Alfred L
authors.initials: AL
authors.lastname: Goldberg
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001324:Autoantigens; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D006868:Hydrolysis; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D007700:Kinetics; D013058:Mass Spectrometry; D051379:Mice; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000378:metabolism*; D010447:Peptide Hydrolases / Q000737:chemistry* / Q000378:metabolism*; D010455:Peptides / Q000737:chemistry; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D011506:Proteins / Q000737:chemistry / Q000378:metabolism*; D011817:Rabbits; D011994:Recombinant Proteins / Q000737:chemistry; D013154:Spleen / Q000378:metabolism; D013997:Time Factors
chemical_list: D001324:Autoantigens; C099558:Ki antigen; D009097:Multienzyme Complexes; D010455:Peptides; D011506:Proteins; D011994:Recombinant Proteins; D007334:Insulin-Like Growth Factor I; D010447:Peptide Hydrolases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; C061553:ATP dependent 26S protease