Studies on rLMP7, a beta-subunit of the multicatalytic proteinase

Subunit compositions and catalytic properties of proteasomes from developmental temperature- sensitive mutants of Drosophila melanogaster

Two dominant temperature-sensitive (DTS) Drosophila mutants are missense mutations of proteasome genes encoding beta-type subunits beta6/C5 (DTS5) and beta2/Z (DTS7). At nonpermissive temperature (29 degrees C), heterozygotes (DTS5/+ and DTS7/+) develop normally until metamorphosis; pupae fail to mature and die before eclosion. Proteasomes were purified from wild-type (WT) and heterozygous adult flies raised at permissive temperature (25 degrees C). Two-dimensional gel electrophoresis separated at least 28 proteins, 13 of which were identified with monospecific antibodies to alpha6/C2 (five species), alpha2/C3 (three species), alpha7/C8 (three species), alpha5/zeta, and beta1/Y subunits. Both quantitative and qualitative differences were observed between WT and DTS/+ proteasomes, with DTS5/+ deviating more from WT than DTS7/+ proteasomes. In DTS5/+ there was a shift to more acidic species of C2 and C3 and a shift to less acidic species of 32-kDa subunits (#3-#7) recognized by an anti-alpha subunit monoclonal antibody (MCP222) and were losses of two 32-kDa subunits (#2 and #3), decreases in Y (25 kDa; 2-fold) and 31-kDa (#9; 2-fold) subunits, and increases in 52-kDa (#1; 1.9-fold) and 24-kDa (#13; 2.3-fold) subunits. In DTS7/+ there was a less pronounced shift to acidic species of C3 and no pI shift in C2 species and subunits #3-#7 and were decreases in #9 (2.5-fold) and #14 (3-fold) and a loss of #2. The three C8 species were similar between WT, DTS5/+, and DTS7/+ proteasomes. Qualitatively, the most dramatic difference was the appearance of a new 24-kDa subunit (#16) in DTS/+ preparations, with about a 14-fold greater amount of #16 in DTS7/+ than in DTS5/+ proteasomes. Catalytically, WT and DTS/+ proteasomes had similar peptidase activities, although the DTS/+ proteasomes were slightly more sensitive to SDS and elevated temperatures in vitro. The incorporation of DTS subunits apparently altered proteasome assembly and/or processing at permissive temperature with little effect on catalytic activities. These data suggest that at nonpermissive temperature, assembly/processing is more severely affected, producing DTS-containing complexes that lack functions essential for cellular proliferation and differentiation at metamorphosis.

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.