Intermediate-type 20 S proteasomes in HeLa cells: Asymmetric subunit composition, diversity and adaptation

The 20 S proteasomes are cylinder-shaped heteromeric dimers with a subunit configuration of alpha 7, beta 7, beta 7, alpha 7. Replacement of the three active site-containing standard beta-subunits (beta 1, beta 2, beta 5) by immuno- -subunits (beta 1i, beta 2i, beta 5i) results in formation of 20 S immuno-proteasomes, while only partial replacement leads to intermediate-type proteasomes. Synthesis of immuno-subunits can be induced by interferon-gamma, which causes a complete transformation of three subtypes of standard proteasomes into three subtypes of intermediate-type proteasomes in HeLa cells, a process that results in a change in the proteolytic activities of the enzymes. HeLa cells producing the proteasome beta 1-subunit tagged with the Fc region-binding ZZ domain of protein A were grown in the presence of interferon-gamma. From these cells, we have purified 20 S proteasomes by using IgG-affinity resin and analysed them by 2D PAGE. Our study showed that subunit replacement can be confined to one half of the proteasome cylinder, resulting in the formation of intermediate-type proteasomes with asymmetric subunit composition. Analysis of proteasomes purified from the cytoplasm, nucleoplasm, and microsomes of HeLa S3 cells reveals that an three compartments are furnished with intermediate-type proteasomes of different subtype and subunit composition, exhibiting different specific proteolytic activities. (C) 2007 Elsevier Ltd. All rights reserved.