Subcellular localization of proteasomes and their regulatory complexes in mammalian cells

Abstract

Proteasomes can exist in several different molecular forms in mammalian cells. The core 20S proteasome, containing the proteolytic sites, binds regulatory complexes at the ends of its cylindrical structure. Together with two 19S ATPase regulatory complexes it forms the 26S proteasome, which is involved in ubiquitin-dependent proteolysis. The 20S proteasome can also bind 11S regulatory complexes (REG, PA28) which play a role in antigen processing, as do the three variable gamma-interferon-inducible catalytic beta-subunits (e.g. LMP7). In the present study, we have investigated the subcellular distribution of the different forms of proteasomes using subunit specific antibodies. Both 20S proteasomes and their 19S regulatory complexes are found in nuclear, cytosolic and microsomal preparations isolated from rat liver. LMP7 was enriched approximately two-fold compared with core alpha-type proteasome subunits in the microsomal preparations. 20S proteasomes were more abundant than 26S proteasomes, both in liver and cultured cell lines. Interestingly, some significant differences were observed in the distribution of different subunits of the 19S regulatory complexes. S12, and to a lesser extent p45, were found to be relatively enriched in nuclear fractions from rat liver, and immunofluorescent labelling of cultured cells with anti-p45 antibodies showed stronger labelling in the nucleus than in the cytoplasm. The REG was found to be localized predominantly in the cytoplasm. Three- to six-fold increases in the level of REG were observed following gamma-interferon treatment of cultured cells but gamma-interferon had no obvious effect on its subcellular distribution. These results demonstrate that different regulatory complexes and subpopulations of proteasomes have different distributions within mammalian cells and, therefore, that the distribution is more complex than has been reported for yeast proteasomes.

Nuclear translocation of antizyme and expression of ornithine decarboxylase and antizyme are developmentally regulated

“…To determine whether nuclear AZ may be involved in the degradation of ODC by nuclear proteasomes (Brooks et al, 2000, and references therein), we blocked proteasomal activity with MG-132 and then analyzed the subcellular localization of ODC. Interestingly, inhibition of proteasome activity in mandibular explants led to nuclear accumulation of ODC in a subset of cells of the surface ectoderm.…”

“…In addition to the previously unknown Az and AZ expression patterns, and novel sites of Odc and ODC expression, we discovered that AZ is present not only in the cytoplasm but also in the nucleus of certain cell types at specific developmental stages. This is interesting in view of the fact that regulatory complexes and subpopulations of proteasomes are present in the nucleus (Brooks et al, 2000). When selectively inhibiting proteasome activity, ODC protein accumulated in the nucleus, suggesting the possible existence of AZ-mediated regulation of ODC in the nucleus by proteasomal degradation.…”

Proteasomal activity‐based probes mark protein homeostasis in muscles

“…This indicates that proteasomal activity in muscle cells varied between cell compartments and changes within cell compartments could alter during cell fusion. Enrichment of proteasomal subunits in the nuclear fraction was previously reported in rat liver and several non‐muscle cells 31. Dynamic sub‐cellular distribution of proteasomal subunits was found in cancer cell cultures, and components of the 19S were enriched in the nuclear fraction 32.…”

Amyloid β-induced FOXRED2 mediates neuronal cell death via inhibition of proteasome activity

“…Proteasomes are localized in the cytoplasm, the nucleus, and the cytoplasmic surface of the ER in mammalian cells [32]. Since FOXRED2 resides in the ER lumen, the occurrence of proteasome inhibition via the direct interaction between the proteasome and FOXRED2 is difficult.…”