Ariadne’s Thread in the Analytical Labyrinth of Membrane Proteins: Integration of Targeted and Shotgun Proteomics for Global Absolute Quantification of Membrane Proteins

From the outer space to the inner cell: deconvoluting the complexity of Bacillus subtilis disulfide stress responses by redox state and absolute abundance quantification of extracellular, membrane, and cytosolic proteins

Understanding cellular mechanisms of stress management relies on omics data as a valuable resource. However, the lack of absolute quantitative data on protein abundances remains a significant limitation, particularly when comparing protein abundances across different cell compartments. In this study, we aimed to gain deeper insights into the proteomic responses of the Gram-positive model bacterium Bacillus subtilis to disulfide stress. We determined proteome-wide absolute abundances, focusing on different sub-cellular locations (cytosol and membrane) as well as the extracellular medium, and combined these data with redox state determination. To quantify secreted proteins in the culture medium, we developed a simple and straightforward protocol for the absolute quantification of extracellular proteins in bacteria. We concentrated extracellular proteins, which are highly diluted in the medium, using StrataClean beads along with a set of standard proteins to determine the extent of the concentration step. The resulting data set provides new insights into protein abundances in different sub-cellular compartments and the extracellular medium, along with a comprehensive proteome-wide redox state determination. Our study offers a quantitative understanding of disulfide stress management, protein production, and secretion in B. subtilis.

IMPORTANCE

Stress responses play a crucial role in bacterial survival and adaptation. The ability to quantitatively measure protein abundances and redox states in different cellular compartments and the extracellular environment is essential for understanding stress management mechanisms. In this study, we addressed the knowledge gap regarding absolute quantification of extracellular proteins and compared protein concentrations in various sub-cellular locations and in the extracellular medium under disulfide stress conditions. Our findings provide valuable insights into the protein production and secretion dynamics of B. subtilis, shedding light on its stress response strategies. Furthermore, the developed protocol for absolute quantification of extracellular proteins in bacteria presents a practical and efficient approach for future studies in the field. Overall, this research contributes to the quantitative understanding of stress management mechanisms and protein dynamics in B. subtilis, which can be used to enhance bacterial stress tolerance and protein-based biotechnological applications.

TerC proteins function during protein secretion to metalate exoenzymes

Cytosolic metalloenzymes acquire metals from buffered intracellular pools. How exported metalloenzymes are appropriately metalated is less clear. We provide evidence that TerC family proteins function in metalation of enzymes during export through the general secretion (Sec-dependent) pathway. Bacillus subtilis strains lacking MeeF(YceF) and MeeY(YkoY) have a reduced capacity for protein export and a greatly reduced level of manganese (Mn) in the secreted proteome. MeeF and MeeY copurify with proteins of the general secretory pathway, and in their absence the FtsH membrane protease is essential for viability. MeeF and MeeY are also required for efficient function of the Mn2+-dependent lipoteichoic acid synthase (LtaS), a membrane-localized enzyme with an extracytoplasmic active site. Thus, MeeF and MeeY, representative of the widely conserved TerC family of membrane transporters, function in the co-translocational metalation of Mn2+-dependent membrane and extracellular enzymes.

Sample Preparation for Mass Spectrometry-Based Absolute Quantification of Bacterial Proteins in Antibiotic Stress Research

Absolute protein quantification is an essential tool for system biology approaches and elucidation of stoichiometry of multi-protein complexes. In this updated chapter, a universal protocol for gel-free absolute protein quantification in bacterial systems is described, which provides adapted methods for cytosolic and membrane proteins. This protocol can be used for sample preparation prior to miscellaneous mass spectrometry-based quantification workflows like AQUA, Hi3, and emPAI. In addition, a focus has been set to the specific challenges in antibiotic stress research.

Redirected Stress Responses in a Genome-Minimized 'midiBacillus' Strain with Enhanced Capacity for Protein Secretion

Genome engineering offers the possibility to create completely novel cell factories with enhanced properties for biotechnological applications. In recent years, genome minimization was extensively explored in the Gram-positive bacterial cell factory Bacillus subtilis, where up to 42% of the genome encoding dispensable functions was removed. Such studies showed that some strains with minimized genomes gained beneficial features, especially for secretory protein production. However, strains with the most minimal genomes displayed growth defects. This focused our attention on strains with less extensive genomic deletions that display close-to-wild-type growth properties while retaining the acquired beneficial traits in secretory protein production. A strain of this category is B. subtilis IIG-Bs27-47-24, here referred to as midiBacillus, which lacks 30.95% of the parental genome. To date, it was unknown how the altered genomic configuration of midiBacillus impacts cell physiology in general, and protein secretion in particular. The present study bridges this knowledge gap through comparative quantitative proteome analyses with focus on protein secretion. Interestingly, the results show that the secretion stress responses of midiBacillus, as elicited by high-level expression of the immunodominant staphylococcal antigen A, are completely different from secretion stress responses that occur in the parental strain 168. We further show that midiBacillus has an increased capacity for translation and that a variety of critical Sec secretion machinery components is present at elevated levels. Altogether, our observations demonstrate that high-level protein secretion has different consequences for wild-type and genome-engineered Bacillus strains, dictated by the altered genomic and proteomic configurations.

IMPORTANCE Our present study showcases a genome-minimized nonpathogenic bacterium, the so-called midiBacillus, as a chassis for the development of future industrial strains that serve in the production of high-value difficult-to-produce proteins. In particular, we explain how midiBacillus, which lacks about one-third of the original genome, effectively secretes a protein of the major human pathogen Staphylococcus aureus that cannot be produced by the parental Bacillus subtilis strain. This is important, because the secreted S. aureus protein is exemplary for a range of targets that can be implemented in future antistaphylococcal immunotherapies. Accordingly, we anticipate that midiBacillus chassis will contribute to the development of vaccines that protect both humans and livestock against diseases caused by S. aureus, a bacterial pathogen that is increasingly difficult to fight with antibiotics, because it has accumulated resistances to essentially all antibiotics that are currently in clinical practice.

Membrane Modulation of Super-Secreting "midiBacillus" Expressing the Major Staphylococcus aureus Antigen - A Mass-Spectrometry-Based Absolute Quantification Approach

Bacillus subtilis has been extensively used as a microbial cell factory for industrial enzymes due to its excellent capacities for protein secretion and large-scale fermentation. This bacterium is also an attractive host for biopharmaceutical production. However, the secretion potential of this organism is not fully utilized yet, mostly due to a limited understanding of critical rearrangements in the membrane proteome upon high-level protein secretion. Recently, it was shown that bottlenecks in heterologous protein secretion can be resolved by genome minimization. Here, we present for the first time absolute membrane protein concentrations of a genome-reduced B. subtilis strain ("midiBacillus") expressing the immunodominant Staphylococcus aureus antigen A (IsaA). We quantitatively characterize the membrane proteome adaptation of midiBacillus during production stress on the level of molecules per cell for more than 400 membrane proteins, including determination of protein concentrations for ∼61% of the predicted transporters. We demonstrate that ∼30% of proteins with unknown functions display a significant increase in abundance, confirming the crucial role of membrane proteins in vital biological processes. In addition, our results show an increase of proteins dedicated to translational processes in response to IsaA induction. For the first time reported, we provide accumulation rates of a heterologous protein, demonstrating that midiBacillus secretes 2.41 molecules of IsaA per minute. Despite the successful secretion of this protein, it was found that there is still some IsaA accumulation occurring in the cytosol and membrane fraction, leading to a severe secretion stress response, and a clear adjustment of the cell's array of transporters. This quantitative dataset offers unprecedented insights into bioproduction stress responses in a synthetic microbial cell.