Differential intra-proteasome interactions involving standard and immunosubunits

Abstract

Animals with immune systems have two types of proteasomes, "standard proteasomes" and "immunoproteasomes" that respectively contain constitutively expressed catalytic subunits or interferon-gamma-inducible catalytic subunits. Interestingly, proteasome assembly is biased against formation of most mixed proteasomes containing combinations of standard subunits and immunosubunits. We previously demonstrated that catalytic subunit propeptide differences contribute to this assembly specificity. In the current study, we investigated the contributions of catalytic subunit propeptides and C-terminal extensions to intra-proteasome protein-protein interactions that are potentially involved in mediating biased assembly of human proteasomes, and we found a number of interactions that differentially depended on these structures. For example, the C-terminal extension of standard subunit beta2 is required for beta2's interaction with adjacent beta3, whereas the C-terminal extension of immunosubunit beta2i is dispensable for beta2i's interaction with beta3. Taken together, our results suggest mechanisms whereby differential intra-proteasome interactions could contribute to proteasome assembly specificity.

Fig. 1

Example of two-hybrid interaction trap assays by yeast co-transformation for interactions between α1-BD and AD fusions to the indicated β2 subunits. Results are included in Table 1, with the scoring system defined in Materials and methods. The first row is a negative control involving α1-BD and empty vector-AD. The bottom row is a positive control involving p53-BD and T-ag-AD. Yeast were grown on SC media lacking tryptophan, leucine, and histidine, and containing 3-AT at the three indicated concentrations.