How Pathogens Maintain Proteostasis During Infection

Area of Science:

• Microbiology
• Molecular Biology
• Cellular Biology

Background:

• Molecular chaperones are essential for maintaining proteostasis by assisting protein folding.
• Cells regulate protein synthesis and folding capacity to prevent toxic aggregate formation.
• Cytoplasmic Mg²⁺ starvation is an infection-relevant stress impacting bacterial proteostasis.

Purpose of the Study:

• To investigate the role of the Hsp70 chaperone DnaK in regulating protein synthesis during Mg²⁺ starvation in *Salmonella enterica* serovar Typhimurium.
• To understand how DnaK's function under stress differs from its canonical roles and from other chaperones like trigger factor.
• To explore the implications of differential chaperone expression in bacterial pathogenesis.

Main Methods:

• Studied the interaction of DnaK with ribosomes in *S. typhimurium* under Mg²⁺-deficient conditions.
• Assessed the effect of DnaK binding on protein synthesis rates.
• Analyzed the expression patterns of DnaK, cochaperones (J-domain proteins, GrpE), and trigger factor under infection-relevant conditions regulated by PhoP.

Main Results:

• DnaK binds to ribosomes and decreases protein synthesis during Mg²⁺ starvation in *S. typhimurium*.
• This inhibitory effect on protein synthesis is independent of J-domain cochaperones and GrpE.
• The virulence regulator PhoP upregulates DnaK but not other chaperones/cochaperones during infection-relevant stress.
• Hsp70 chaperone activity at the ribosome differs between bacteria (inhibition) and eukaryotes (promotion) and varies with growth conditions.

Conclusions:

• DnaK plays a distinct role in regulating protein synthesis, separate from its folding functions, during Mg²⁺ starvation.
• Differential expression of chaperones and cochaperones, orchestrated by PhoP, contributes to *S. typhimurium* pathogenesis.
• Hsp70 chaperone activity at the ribosome is context-dependent, varying with cellular conditions and organismal type, highlighting functional divergence.