Protein homeostasis is fundamental to cell function and survival. It relies on an interconnected network of processes involving protein synthesis, folding, post-translational modification and degradation as well as regulators of these processes. Here we provide an update on the roles, regulation and subcellular localization of the protein homeostasis machinery in the Gram-positive model organism Bacillus subtilis. We discuss emerging ideas and current research gaps in the field that, if tackled, increase our understanding of how Gram-positive bacteria, including several human pathogens, maintain protein homeostasis and cope with stressful conditions that challenge their survival.

To survive under adverse and fluctuating conditions, bacteria have evolved several mechanisms that sense and respond to stress. In the gram-positive bacterium Bacillus subtilis, the Spx protein (encoded by spxA) is a global transcriptional regulator controlling the expression of large number of genes in response to oxidative stress triggered by thiol-specific agents (Nakano et al., 2003;Rochat et al., 2012).Recent studies have shed light on the role of Spx in responding to other stressors, such as heat shock and cell wall-active compounds, in addition to its well-established role in the disulfide stress response (Rojas-Tapias & Helmann, 2018a;Runde et al., 2014;. Thus, Spx is a versatile and important regulator that mediates the B. subtilis response to various stressors. Spx modulates transcription of target genes by directly interacting with the C-terminal domain of the α-subunit of RNA polymerase (αCTD) (

The protein homeostasis network ensures a proper balance between synthesis, folding, and degradation of all cellular proteins. DnaK and trigger factor (TF) are ubiquitous bacterial molecular chaperones that assist in protein folding, as well as preventing protein misfolding and aggregation. In Escherichia coli, DnaK and TF possess partially overlapping functions. Their combined depletion results in proteostasis collapse and is synthetically lethal at temperatures above 30°C. To increase our understanding on how proteostasis is maintained in Gram-positive bacteria, we have investigated the physiological effects of deleting dnaK and tig (encoding for DnaK and TF) in Bacillus subtilis. We show that combined deletion of dnaK and tig in B. subtilis is non-lethal, but causes a severe pleiotropic phenotype, including an aberrant twisted and filamentous cell morphology, as well as decreased tolerance to heat and to cell wall active antibiotics and hydrolytic enzymes, indicative of defects in cell wall integrity. In addition, cells lacking DnaK and TF have a much smaller colony size due to defects in motility. Despite these physiological changes, we observed no major compromises in important cellular processes such as cell growth, FtsZ localization and division and only moderate defects in spore formation. Finally, through suppressor analyses, we found that the wild-type cell shape can be partially restored by mutations in genes involved in metabolism or in other diverse cellular processes.