Intracellular foreign or aberrant host proteins are cleaved into peptide fragments of a precise size, such that they can be loaded on to class I major histocompatibility complex (MHC I) molecules and presented externally to cytotoxic CD8+ T lymphocytes (CTLs) (reviewed by Embgenbroich M & Burgdorf S 2018). In the cytosol, CTL-recognized peptides are generated by proteasomes, which exist as a mixture of compositionally distinct complexes (reviewed by Kloetzel PM 2004; Habib JA et al., 2022). The proteasome complex consists of 20S proteolytic core particle (CP) associated with a regulatory particle (RP) such as 19S RP or PA28 RP (Zhao J et al. 2022; Adolf F et al., 2024; reviewed by Watanabe A et al., 2022). The catalytic subunits PSMB6 (β1), PSMB7 (β2), and PSMB5 (β5) within 20S CP exhibit caspase-like, trypsin-like, and chymotrypsin-like activities, respectively (Vigneron N et al. 2015; Rut W & Drag M 2016). During peptide bond cleavage, threonine from the active site of the catalytic β-subunit binds to the N-terminal peptide fragment forming an unstable acyl-enzyme intermediate, where a peptide fragment remains attached to the proteasome. Subsequently, a water molecule hydrolyzes the bond between the peptide and the proteasome, resulting in the release of the peptide's C-terminal end (Mishto M et al. 2012; Ebstein F et al. 2016; Soh WT et al., 2024). Either one or two regulatory particles (RP) can attach to the 20S CP (Zhao J et al. 2022; Adolf F et al., 2024; reviewed by Watanabe A et al., 2022). The 19S RP binds 20S CP to form the 26S proteasome, which degrades both foreign and self-proteins in the ubiquitin (Ub)- and ATP-dependent manner (Zhu Y et al., 2018; Dong Y et al., 2019; Zhang S et al., 2022; reviewed by Sahu I & Glickman MH 2021a). In addition, a subset of intracellular proteins, especially intrinsically disordered proteins, can directly bind to 20S CP for Ub- and ATP-independent proteasomal degradation (Makaros Y et al., 2023; Mamrosh JL et al., 2023; Pepelnjak M et al., 2024; reviewed by Sahu I & Glickman MH 2021 b; Bialek W et al., 2023). Unlike 19S, the PA28 regulatory particle does not have a ubiquitin receptor; binding of PA28 to the 20S CP promotes Ub-, ATP-independent protein breakdown. PA28 proteasomes are thought to be important for rapid degradation of misfolded proteins under conditions of oxidative stress or degradation of proteins with intrinsically disordered regions (Habib JA et al., 2020; Chen J et al., 2021; Zhao J et al. 2022; reviewed by Thomas T et al. 2023). Various immune cells, such as T cells, B cells, and antigen-presenting cells, constitutively express a specialized form of proteasomes, known as immunoproteasomes, in which the catalytic subunits PSMB6 (β1), PSMB7 (β2), and PSMB5 (β5) of 20S CP are substituted with PSMB9 (β1i), PSMB10 (β2i), and PSMB8 (β5i), respectively (Shin EC et al., 2006; Bai M et al., 2014; Santos R et al., 2017). This variation of the core particle is known as 20S iCP. Immunoproteasomes utilize the PA28 regulatory particle to degrade antigens (Lesne J et al., 2020; Chen J et al., 2021). Under inflammatory conditions, the expression of the catalytic and PA28 regulatory subunits of immunoproteasome is induced by cytokines like interferon-gamma (IFNγ), type I interferons, and tumor necrosis factor alpha (TNF-α). Immunoproteasomes possess enhanced chymotrypsin- and trypsin-like activities, alongside with the reduced caspase-like activity. These distinct enzymatic properties of immunoproteasomes generate substrate cleavage patterns that enhance loading of peptides onto the class I MHC for immune presentation to CTL (reviewed by Tomko RJ and Hochstrasser M 2013).
This Reactome module describes Ub-independent antigen processing by PA28-20S CP and PA28-20S iCP proteasomes.
