title: Isolation and characterization of two 20S proteasomes from the endoplasmic reticulum of rat liver microsomes.
abstract: Two new forms of proteasomes, designated as the endoplasmic reticulum (ER) membrane-associated proteasome (ERa proteasome) and ER membrane-bound proteasome (ERb proteasome), were purified to homogeneity from 0.0125 and 2.5% sodium cholate extracts, respectively, of a rat liver microsomal fraction. SDS-PAGE analysis revealed that the purified ERa and ERb proteasomes were composed of multiple subunits similar to the cytosolic 20S proteasome. However, electrophoretic, structural and immunochemical differences between the ERa, ERb and cytosolic 20S proteasomes were observed on native PAGE, two-dimensional (2D) PAGE, and immunoblot analyses. Purification of ERb from a 2.5% sodium cholate extract of the trypsin-treated microsomal fraction yielded a trypsin-modified form of ERb (tERb), which lacked the C2 subunit at least. On the other hand, no ERa proteasome was obtained from the 0.0125% sodium cholate extract of the trypsin-treated microsomes, suggesting that ERa and ERb are ER membrane-associated and -bound proteasomes, respectively. The ERa, ERb, and cytosolic 20S proteasomes exhibited similar specificities as to peptide hydrolyzing activity, although differences in their activities were noted in the presence of SDS and phospholipid. With respect to the proteolysis of protein substrates, only the ERb proteasome cleaved beta-casein, and it also degraded reduced and carboxymethylated lysozyme considerably faster than the cytosolic 20S and ERa proteasomes. Collectively our results suggest that the ERa and ERb proteasomes may play roles in intracellular proteolysis distinct from that of the cytosolic 20S proteasome.
authors.affiliation: Department of Life Science, Faculty of Science, Himeji Institute of Technology, Harima Science Garden City, Hyogo, 678-1297, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Hori
authors.forename: T
authors.initials: T
authors.lastname: Nembai
authors.forename: Y
authors.initials: Y
authors.lastname: Miyata
authors.forename: T
authors.initials: T
authors.lastname: Hayashi
authors.forename: K
authors.initials: K
authors.lastname: Ueno
authors.forename: T
authors.initials: T
authors.lastname: Koide
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D003546:Cysteine Endopeptidases / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism; D015180:Electrophoresis, Gel, Two-Dimensional; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D009097:Multienzyme Complexes / Q000737:chemistry / Q000302:isolation & purification* / Q000378:metabolism; D009842:Oligopeptides / Q000737:chemistry; D011480:Protease Inhibitors / Q000494:pharmacology; D046988:Proteasome Endopeptidase Complex; D051381:Rats; D017207:Rats, Sprague-Dawley; D020358:Sodium Cholate; D013379:Substrate Specificity
chemical_list: D009097:Multienzyme Complexes; D009842:Oligopeptides; D011480:Protease Inhibitors; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex; D020358:Sodium Cholate