Loss-of-Function Mutations in HspR Rescue the Growth Defect of a Mycobacterium tuberculosis Proteasome Accessory Factor E ( pafE ) Mutant.

Mycobacterium tuberculosis uses a proteasome to degrade proteins by both ATP-dependent and -independent pathways. While much has been learned about ATP-dependent degradation, relatively little is understood about the ATP-independent pathway, which is controlled by Mycobacterium tuberculosis proteasome accessory factor E (PafE). Recently, we found that a Mycobacterium tuberculosis pafE mutant has slowed growth in vitro and is sensitive to killing by heat stress. However, we did not know if these phenotypes were caused by an inability to degrade the PafE-proteasome substrate HspR (heat shock protein repressor), an inability to degrade any damaged or misfolded proteins, or a defect in another protein quality control pathway. To address this question, we characterized pafE suppressor mutants that grew similarly to pafE + bacteria under normal culture conditions. All but one suppressor mutant analyzed contained mutations that inactivated HspR function, demonstrating that the slowed growth and heat shock sensitivity of a pafE mutant were caused primarily by the inability of the proteasome to degrade HspR. IMPORTANCE Mycobacterium tuberculosis encodes a proteasome that is highly similar to eukaryotic proteasomes and is required for virulence. We recently discovered a proteasome cofactor, PafE, which is required for the normal growth, heat shock resistance, and full virulence of M. tuberculosis In this study, we demonstrate that PafE influences this phenotype primarily by promoting the expression of protein chaperone genes that are necessary for surviving proteotoxic stress.