Resolving hidden stoichiometries in Bacterial proteasome activator (Bpa)-substrate complexes by cryo-EM and charge detection mass spectrometry

Bpa (Bacterial proteasome activator) is a regulatory particle within the Mycobacterium tuberculosis (Mtb) proteasome system that that facilitates ATP-independent substrate engagement and delivery to the 20S core particle (CP) for degradation. The best characterized Bpa substrate is HspR, a transcriptional repressor of Mtb stress-response genes whose Bpa-dependent degradation is required for pathogen virulence. However, the stoichiometry of the Bpa:HspR complex, the molecular mechanism of substrate engagement, and the heterogeneity of the resulting assemblies remain unclear. Here, we combine charge detection mass spectrometry (CDMS) and single-particle electron cryomicroscopy (cryo-EM) as complementary approaches to characterize both apo and HspR-bound Bpa. CDMS revealed a previously unreported undecameric apo species and defines a Bpa12:HspR2 complex stoichiometry with minimal heterogeneity. Cryo-EM, performed without the employment of cross-linking reagents, independently confirmed the presence of undecameric Bpa in solution and localized substrate-associated density to the C-terminal H4 helix of Bpa. Together, these complementary single-particle approaches inform future efforts to target the Mtb proteasome system and provide new molecular insight into proteasomal substrate recognition in prokaryotes. Table of Content Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=103 SRC="FIGDIR/small/722288v1_ufig1.gif" ALT="Figure 1"> View larger version (34K): org.highwire.dtl.DTLVardef@be4bb6org.highwire.dtl.DTLVardef@15ccf73org.highwire.dtl.DTLVardef@379a2eorg.highwire.dtl.DTLVardef@6b7b90_HPS_FORMAT_FIGEXP M_FIG C_FIG