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.