Proteostasis in Bacillus subtilis : chaperones and stress-response mechanisms

Abstract: The maintenance of protein homeostasis, also referred to as proteostasis, is essential for preserving the structural and functional stability of the proteome in living organisms. This involves the precise control of protein synthesis, folding, and degradation. As proteostasis disruption can cause growth and survival deficiencies in bacteria, the investigation of proteostasis has rapidly emerged as a growing field of study, and targeting proteostasis has become an attractive strategy for combating bacterial infections.In this thesis proteostasis was studied in the Gram-positive model organism Bacillus subtilis. The research primarily concentrated on two distinct aspects of proteostasis: chaperones (which are highly conserved protein-folding “machines” that play a central role in maintaining proteostasis) and response mechanisms to proteotoxic stress. Previous studies have extensively examined the role of chaperones and their effects on cell function upon removal in the Gram-negative bacterial model Escherichia coli. Research in other kinds of bacteria is needed. The findings presented in Paper I build on our current knowledge about proteostasis in bacteria by showing the pleotropic phenotype caused by the combined removal of the DnaK and trigger factor chaperones in B. subtilis. The effects include altered cell morphology, thermotolerance, and cell wall integrity. Paper I also shows how B. subtilis can partially adapt to chaperone deficiency by acquiring second-site suppressor mutations in genes required for different biological processes. Paper II shows that chaperone removal not only affects planktonic cells, but also influences B. subtilis multicellularity (i.e., formation of biofilm communities). Mutant biofilms are architecturally aberrant and display an altered presence of cell types. The second part of my study was on the function of the Spx-YjbH system in coping with disulfide stress (a proteotoxic stress condition). Spx is an important transcriptional regulator of the proteotoxic stress response; YjbH is an adaptor protein for Spx, whose levels are regulated by stress-induced protein aggregation. Paper III provides insights into this system by studies at a single cell level, with special focus on the localization, dynamics and inheritance of YjbH aggregates. It also concerns the contribution of the different YjbH protein domains to aggregation function and YjbH orthologs of the Gram-positive bacteria Staphylococcus aureus and Listeria monocytogenes.