title: Spatial arrangement and functional role of α subunits of proteasome activator PA28 in hetero-oligomeric form.
abstract: A major form of proteasome activator PA28 is a heteroheptamer composed of interferon-γ-inducible α and β subunits, which share approximately 50% amino acid identity and possess distinct insert loops. This activator forms a complex with the 20S proteasome and thereby stimulates proteasomal degradation of peptides in an ATP-independent manner, giving rise to smaller antigenic peptides presented by major histocompatibility complex class I molecules. In this study, we performed biophysical and biochemical characterization of the structure and function of the PA28 hetero-oligomer. Deuteration-assisted small-angle neutron scattering demonstrated three α and four β subunits are alternately arranged in the heptameric ring. In this arrangement, PA28 loops surround the central pore of the heptameric ring (site for peptide entry). Activating the 20S proteasome with a PA28 mutant that lacked the α subunit loops cleaved model substrates longer than a nonapeptide with better efficiency when compared to wild-type PA28. Based on these data, we hypothesize that the flexible PA28 loops act as gatekeepers, which function to select the length of peptide substrates to be transported between the proteolytic chamber and the extra-proteasomal medium.
authors: Research Reactor Institute, Kyoto University, Osaka 590-0494, Japan. sugiyama@rri.kyoto-u.ac.jp
authors: Masaaki
authors: M
authors: Sugiyama
authors: Hiroki
authors: H
authors: Sahashi
authors: Eiji
authors: E
authors: Kurimoto
authors: Shin-ichi
authors: S
authors: Takata
authors: Hirokazu
authors: H
authors: Yagi
authors: Keita
authors: K
authors: Kanai
authors: Eri
authors: E
authors: Sakata
authors: Yasufumi
authors: Y
authors: Minami
authors: Keiji
authors: K
authors: Tanaka
authors: Koichi
authors: K
authors: Kato
mesh_terms: D000818:Animals; D051379:Mice; D010455:Peptides / Q000737:chemistry; D046988:Proteasome Endopeptidase Complex / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation; D055503:Protein Multimerization; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D059748:Proteolysis
chemical_list: D010455:Peptides; D021122:Protein Subunits; D046988:Proteasome Endopeptidase Complex; C486213:Psme1 protein, mouse