Both, constitutive (c20S) as well as immunoproteasomes (i20S) are exclusively formed de novo following a sophisticated and not yet fully understood biogenesis program involving the biosynthesis of all subunits, their assembly and maturation processes.
The first step is the concerted expression of all 7 α, the proforms of all 7 β subunits and helper factors. Mammalian cells are able to compensate for the inhibition of intracellular proteasomal activity by de novo formation of proteasome complexes in a positive autoregulatory feed-back loop. Importantly, biogenesis of eukaryotic 20S complexes is assisted by helper factors. The assembly process is initiated by the formation of a heptameric α-ring, whose formation is supported by the assembly chaperones Pac1 and Pac2 acting as a heterodimer. The α-ring serves as a docking matrix for the sequential association of the β-subunit proforms, resulting in a distinct 13S assembly intermediate. The proteasome maturation protein POMP is essentially required for coordination of the recruitment of the β-subunit proforms and their processing by a cis- and trans autocatalysis. Final activation of the β-subunits requires the formation of the preholoproteasome assembly intermediate by dimerization of two hemiproteasome intermediates that undergo structural rearrangements. Concomitantly, the cis– and trans-autocatalytic removal of the β-subunit-propeptides liberates the active site threonines of the now fully active 20S core proteasome. At this stage the degradation of POMP signals the successful completion of the proteasome biogenesis program. Assembly of i20S basically follows the same principle after the coincident induction of POMP and the three immunosubunits β1i, β2i and β5i by Interferon-γ. The proteasome inhibitor PI31 can modulate this process. The dynamics of this process is determined by the rapid activation of β5i/ LMP7 and the immediate β5i/LMP7-dependent degradation of POMP. The molecular interplay between POMP and the proteasomal β5i subunit LMP7 essentially accelerate immunoproteasome biogenesis compared to constitutive 20S proteasome assembly nearly four-fold. Moreover, i20S formation is a transient response, since immunoproteasomes exhibit a considerably shortened half-life compared to constitutive proteasomes. This rapid adaptation of the proteasome system to an immunological challenge allows cells to return rapidly to a normal situation once an infection is cleared. Future projects will focus on the dynamics of proteasome formation in early phases after cytokine treatment and its impact on MHC class I epitope generation. The main goal of an additional project is the analysis of the molecular interactions in their structural, functional and cellular dynamics which control 20S proteasome biogenesis in mammalian cells.

Selective Publications

Tumor cell lines expressing the proteasome subunit isoform LMP7E1 exhibit immunoproteasome deficiency. Cancer Res. 2006 66(2):649-52.

Interferon-gamma, the functional plasticity of the ubiquitin-proteasome system, and MHC class I antigen processing. Immunol Rev. 2005 207:19-30.

IFN-gamma-induced immune adaptation of the proteasome system is an accelerated and transient response. Proc Natl Acad Sci USA 102, 9241-6 (2005).

Rearrangement of the 16S precursor subunits is essential for the formation of the active 20S proteasome. Biophys J. 2004 87(6):4098-105.

Inhibition of proteasome activity induces concerted expression of proteasome genes and de novo formation of Mammalian proteasomes. J Biol Chem 278, 21517-25 (2003).

The components of the proteasome system and their role in MHC class I antigen processing. Rev Physiol Biochem Pharmacol 148, 81-104 (2003).

PI31 is a modulator of proteasome formation and antigen processing. Proc Natl Acad Sci USA 99, 14344-9 (2002).

20S proteasome biogenesis. Biochimie 83, 289-93 (2001).

Characterisation of the newly identified human Ump1 homologue POMP and analysis of LMP7(beta 5i) incorporation into 20 S proteasomes. J Mol Biol 301, 1-9 (2000).