SUBPOPULATIONS OF PROTEASOMES IN RAT-LIVER NUCLEI, MICROSOMES AND CYTOSOL

Mammalian proteasomes are composed of 14-17 different types of subunit s, some of which, including major-histocompatibility-complex-encoded s ubunits LMP2 and LMP7, are non-essential and present in variable amoun ts. We have investigated the distribution of total proteasomes and som e individual subunits in rat liver by quantitative immunoblot analysis of purified subcellular fractions (nuclei, mitochondria, microsomes a nd cytosol). Proteasomes were mainly found in the cytosol but were als o present in the purified nuclear and microsomal fractions. In the nuc lei, proteasomes were soluble or loosely attached to the chromatin, si nce they could be easily extracted by treatment with nucleases or high concentrations of salt. In the microsomes, proteasomes were on the ou tside of the membranes. Further subfractionation of the microsomes sho wed that the proteasomes in this fraction were associated with the smo oth endoplasmic reticulum and with the cis-Golgi but were practically absent from the rough endoplasmic reticulum. Using monospecific antibo dies for some proteasomal subunits (C8, C9, LMP2 and Z), the compositi on 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 rela tive amounts of some beta subunits varied. Subunit Z was enriched in n uclear proteasomes but low in microsome-asssociated proteasomes, where as LMP2, which was relatively low in nuclei, showed a small enrichment in the microsomes. These differences in subunit composition of protea somes probably reflect differences in the function of proteasomes in d istinct cell compartments.