title: Subpopulations of proteasomes in rat liver nuclei, microsomes and cytosol.
abstract: Mammalian proteasomes are composed of 14-17 different types of subunits, some of which, including major-histocompatibility-complex-encoded subunits LMP2 and LMP7, are non-essential and present in variable amounts. We have investigated the distribution of total proteasomes and some individual subunits in rat liver by quantitative immunoblot analysis of purified subcellular fractions (nuclei, mitochondria, microsomes and cytosol). Proteasomes were mainly found in the cytosol but were also present in the purified nuclear and microsomal fractions. In the nuclei, proteasomes were soluble or loosely attached to the chromatin, since they could be easily extracted by treatment with nucleases or high concentrations of salt. In the microsomes, proteasomes were on the outside of the membranes. Further subfractionation of the microsomes showed that the proteasomes in this fraction were associated with the smooth endoplasmic reticulum and with the cis-Golgi but were practically absent from the rough endoplasmic reticulum. Using monospecific antibodies for some proteasomal subunits (C8, C9, LMP2 and Z), the composition of proteasomes in nuclei, microsomes and cytosol was investigated. Although there appear not to be differences in proteasome composition in the alpha subunits (C8 and C9) in the different locations, the relative amounts of some beta subunits varied. Subunit Z was enriched in nuclear proteasomes but low in microsome-associated proteasomes, whereas LMP2, which was relatively low in nuclei, showed a small enrichment in the microsomes. These differences in subunit composition of proteasomes probably reflect differences in the function of proteasomes in distinct cell compartments.
authors.affiliation: Instituto de Investigaciones Citológicas, Valencia, Spain.
authors.forename: A
authors.initials: A
authors.lastname: Palmer
authors.forename: A J
authors.initials: AJ
authors.lastname: Rivett
authors.forename: S
authors.initials: S
authors.lastname: Thomson
authors.forename: K B
authors.initials: KB
authors.lastname: Hendil
authors.forename: G W
authors.initials: GW
authors.lastname: Butcher
authors.forename: G
authors.initials: G
authors.lastname: Fuertes
authors.forename: E
authors.initials: E
authors.lastname: Knecht
mesh_terms: D000818:Animals; D000906:Antibodies; D002458:Cell Fractionation; D002467:Cell Nucleus / Q000201:enzymology; D003546:Cysteine Endopeptidases / Q000032:analysis* / Q000276:immunology; D003593:Cytoplasm / Q000201:enzymology; D004591:Electrophoresis, Polyacrylamide Gel; D004721:Endoplasmic Reticulum / Q000201:enzymology; D005455:Fluorescent Antibody Technique; D006056:Golgi Apparatus; D015151:Immunoblotting; D007124:Immunoenzyme Techniques; D008099:Liver / Q000201:enzymology* / Q000648:ultrastructure; D008854:Microscopy, Electron; D008862:Microsomes, Liver / Q000201:enzymology; D009097:Multienzyme Complexes / Q000032:analysis* / Q000276:immunology; D046988:Proteasome Endopeptidase Complex; D011487:Protein Conformation; D051381:Rats; D014357:Trypsin / Q000378:metabolism
chemical_list: D000906:Antibodies; D009097:Multienzyme Complexes; D014357:Trypsin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex