Hybrid proteasomes - Induction by interferon-gamma and contribution to ATP-dependent proteolysis

Eukaryotic cells contain various types of proteasomes. Core 20 S proteasome s (abbreviated 20 S below) have two binding sites for the regulatory partic les, PA700 and PA28. PA700-20 S-PA700 complexes are known as 26 S proteasom es and are ATP-dependent machines that degrade cell proteins. PA28 is found both in previously described complexes of the type PA28-20 S-PA28 and in c omplexes that also contain PA700, as PA700-20 S-PA28. We refer to the latte r as "hybrid proteasomes." The relative amounts of the various types of pro teasomes in HeLa extracts were determined by a combination of immunoprecipi tation and immunoblotting. Hybrid proteasomes accounted for about a fourth of all proteasomes in the extracts. Association of PA28 and proteasomes pro ved to be ATP-dependent. Hybrid proteasomes catalyzed ATP-dependent degrada tion of ornithine decarboxylase (ODC) without ubiquitinylation, as do 26 S proteasomes. In contrast, the homo-PA28 complex (PA28-20 S-PA28) was incapa ble of degrading ODC. Intriguingly, a major immunomodulatory cytokine, inte rferon-gamma, appreciably enhanced the ODC degradation in HeLa and SW620 ce lls through induction of the hybrid proteasome, which may also be responsib le for the immunological processing of intracellular antigens. Taken togeth er, we report here for the first time the existence of two types of ATP-dep endent proteases, the 26 S proteasome and the hybrid proteasome, which appe ar to share the ATP-dependent proteolytic pathway in mammalian cells.