Although proteasomes are abundant in the nucleoplasm little is known of proteasome-dependent proteolysis within the nucleus. Thus, we monitored the subcellular distribution of nuclear proteins in correlation with proteasomes. The proteasomal pathway clears away endogenous proteins, regulates numerous cellular processes, and delivers immunocompetent peptides to the antigen presenting machinery. Confocal laser scanning microscopy revealed that histones, splicing factor SC35, spliceosomal components, such as U1-70k or SmB/B', and PML partially colocalize with 20S proteasomes in nucleoplasmic substructures, whereas the centromeric and nucleolar proteins topoisomerase 1, fibrillarin, and UBF did not overlap with proteasomes. The specific inhibition of proteasomal processing with lactacystin induced accumulation of histone protein H2A, SC35, spliceosomal components, and PML, suggesting that these proteins are normally degraded by proteasomes. In contrast, concentrations of centromeric proteins CENP-B and -C and nucleolar proteins remained constant during inhibition of proteasomes. Quantification of fluorescence intensities corroborated that nuclear proteins which colocalize with proteasomes are degraded by proteasome-dependent proteolysis within the nucleoplasm. These data provide evidence that the proteasome proteolytic pathway is involved in processing of nuclear components, and thus may play an important role in the regulation of nuclear structure and function. (C) 2002 Elsevier Science (USA). All rights reserved.