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.

Genes of the LMP/TAP cluster are associated with the human autoimmune disease vitiligo

“…The proteasome usually consists of at least 14 different subunits, three of which have catalytic sites. 37 Upon IFNg stimulation, the constitutive catalytic subunits b5, B2 and b1 are replaced with three IFN-g-inducible subunits, LMP2 (b1i), LMP7 (b5i) and MECL1 (b2i), creating the immunoproteasome. The different patterns of peptide cleavage by the immunoproteasome produce peptides that are more efficiently transported by the TAP complex and have improved binding to the antigen-binding cleft of MHC class I molecules.…”

Different anti-aggregation and pro-degradative functions of the members of the mammalian sHSP family in neurological disorders

“…In fact, while the ubiquitin proteasome system is present and active both in the nucleus and in the cytoplasm, autophagy (which can be facilitated by some HSPB members; see earlier) is confined to the cytoplasm [123,124]. HSPB8 and HSPB6, which mainly rely on autophagy, are expected to mainly act on cytoplasmic aggregating proteins.…”

Ubiquitin-independent p53 proteasomal degradation

“…22,23 Free 20S core particles constitute a major portion of the total amount of proteasomes and are present both in the nucleus and cytoplasm of the cell. 24 Interestingly, NQO1 co-fractionates with the 20S core particle but not the 26S proteasome. 17 As NQO1 inhibits p53 degradation by default, this finding provided the first evidence that the 20S but not the 26S proteasome regulates this process.…”