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Sequence information within proteasomal prosequences mediates efficient integration of beta-subunits into the 20 S proteasome complex.

The maturation of proteases is governed by prosequences. During the biogenesis of the highly oligomeric eukaryotic 20 S proteasome five different prosequence-containing subunits have to be integrated and processed either by autocatalysis or by neighbouring subunits. To analyse the functional impact of proteasomal prosequences during complex formation, the propeptide of the facultative subunit beta1i/LMP2 was truncated to nine amino acid residues or completely deleted. Additionally, the charged residues within the truncated beta1i/LMP2 version were replaced by neutral residues. While deletion did not affect subunit incorporation, the presence of charged residues within the truncated version of the LMP2 propeptide diminished incorporation efficiency, an effect that was restored upon replacement of the charged amino acids against neutral components. During immunoproteasome formation, incorporation and processing of inducible proteasome beta-subunits are cooperative processes. We demonstrate a linear correlation of the levels of beta2i/MECL1 and beta1i/LMP2 within 20 S proteasomes, suggesting a physical interaction to be the molecular basis for the biased incorporation of both subunits. In the absence of beta5i/LMP7, precursor complexes containing unprocessed beta1i/LMP2 accumulated. The contribution of beta5i/LMP7 on the cooperative formation of a homogeneous population of immunoproteasome is therefore most likely based on an acceleration of the beta1i/LMP2 and potentially of beta2i/MECL1 processing kinetics.