Protease-Mediated Protein Quality Control for Bacterial Acid Resistance

Exploring Host-Guest Interactions within a 600 kDa DegP Protease Cage Complex Using Hydrodynamics Measurements and Methyl-TROSY NMR

The DegP protease-chaperone operates within the periplasm of Gram-negative bacteria, where it assists in the regulation of protein homeostasis, promotes virulence, and is essential to survival under stress. To carry out these tasks, DegP forms a network of preorganized apo oligomers that facilitate the capture of substrates within distributions of cage-like complexes which expand to encapsulate clients of various sizes. Although the architectures of DegP cage complexes are well understood, little is known about the structures, dynamics, and interactions of client proteins within DegP cages and the relationship between client structural dynamics and function. Here, we probe host-guest interactions within a 600 kDa DegP cage complex throughout the DegP activation cycle using a model α-helical client protein through a combination of hydrodynamics measurements, methyl-transverse relaxation optimized spectroscopy-based solution nuclear magnetic resonance studies, and proteolytic activity assays. We find that in the presence of the client, DegP cages assemble cooperatively with few intermediates. Our data further show that the N-terminal half of the bound client, which projects into the interior of the cages, is predominantly unfolded and flexible, and exchanges between multiple conformational states over a wide range of time scales. Finally, we show that a concerted structural transition of the protease domains of DegP occurs upon client engagement, leading to activation. Together, our findings support a model of DegP as a highly cooperative and dynamic molecular machine that stabilizes unfolded states of clients, primarily via interactions with their C-termini, giving rise to efficient cleavage.

YAP/TEAD-induced PRIM1 contributes to the progression and poor prognosis of gastric carcinoma

Gastric carcinoma has a poor prognosis and low survival rate. PRIM1 is closely associated with the origin of DNA replication and serves as a carcinogenic factor in multiple tumors. This study aimed to explore the functions of PRIM1 in the progression of gastric carcinoma. The luciferase reporter assay examined the regulatory effect of YAP1/TEAD4 on PRIM1. A xenograft tumor mouse model was constructed to observe cancer cell proliferation in vivo. The upregulation of PRIM1 was found in gastric carcinoma cells and tissues, and it was associated with poor prognosis. Silencing PRIM1 inhibited cell proliferation, arrested the cell cycle, and upregulated Cdc25, Cyclin B, and Cdc2 expression. In addition, apoptosis was increased upon PRIM1 knockdown, accompanied by increased protein levels of cleaved caspase-3 and caspase-8. In vivo, knockdown of PRIM1 suppressed the growth of xenograft tumors formed by gastric carcinoma cells. Moreover, PRIM1 silencing elevated the chemosensitivity of gastric carcinoma cells. By investigating molecular events downstream of the Hippo signaling pathway, we found that PRIM1 was a target gene of the YAP1/TEAD4 transcriptional regulatory complex. PRIM1 represents a novel target for gastric carcinoma therapeutic approaches.

Making a chink in their armor: Current and next-generation antimicrobial strategies against the bacterial cell envelope

Gram-negative bacteria are uniquely equipped to defeat antibiotics. Their outermost layer, the cell envelope, is a natural permeability barrier that contains an array of resistance proteins capable of neutralizing most existing antimicrobials. As a result, its presence creates a major obstacle for the treatment of resistant infections and for the development of new antibiotics. Despite this seemingly impenetrable armor, in-depth understanding of the cell envelope, including structural, functional and systems biology insights, has promoted efforts to target it that can ultimately lead to the generation of new antibacterial therapies. In this article, we broadly overview the biology of the cell envelope and highlight attempts and successes in generating inhibitors that impair its function or biogenesis. We argue that the very structure that has hampered antibiotic discovery for decades has untapped potential for the design of novel next-generation therapeutics against bacterial pathogens.

Comparative Proteomic Analysis Revealing ActJ-Regulated Proteins in Sinorhizobium meliloti

To adapt to different environmental conditions, Sinorhizobium meliloti relies on finely tuned regulatory networks, most of which are unexplored to date. We recently demonstrated that deletion of the two-component system ActJK renders an acid-vulnerable phenotype in S. meliloti and negatively impacts bacteroid development and nodule occupancy as well. To fully understand the role of ActJ in acid tolerance, S. meliloti wild-type and S. meliloti ΔactJ proteomes were compared in the presence or absence of acid stress by nanoflow ultrahigh-performance liquid chromatography coupled to mass spectrometry. The analysis demonstrated that proteins involved in the synthesis of exopolysaccharides (EPSs) were notably enriched in ΔactJ cells in acid pH. Total EPS quantification further revealed that although EPS production was augmented at pH 5.6 in both the ΔactJ and the parental strain, the lack of ActJ significantly enhanced this difference. Moreover, several efflux pumps were found to be downregulated in the ΔactJ strain. Promoter fusion assays suggested that ActJ positively modulated its own expression in an acid medium but not at under neutral conditions. The results presented here identify several ActJ-regulated genes in S. meliloti, highlighting key components associated with ActJK regulation that will contribute to a better understanding of rhizobia adaptation to acid stress.

Function, molecular mechanisms, and therapeutic potential of bacterial HtrA proteins: An evolving view

Members of the high temperature requirement A (HtrA) protein family are widely distributed amongst prokaryotic and eukaryotic species. HtrA proteins have ATP-independent dual chaperone-protease activity and mediate protein quality control. Emerging evidence indicates that HtrA family members are vital for establishing infections and bacterial survival under stress conditions. Bacterial HtrA proteins are increasingly thought of as important new targets for antibacterial drug development. Recent literature suggests that HtrA protein AlgW from Pseudomonas aeruginosa has distinct structural, functional, and regulatory characteristics. The novel dual-signal activation mechanism seen in AlgW is required to modulate stress and drug responses in bacteria, prompting us to review our understanding of the many HtrA proteins found in microorganisms. Here, we describe the distribution of HtrA gene orthologues in pathogenic bacteria, discuss their structure–function relationships, outline the molecular mechanisms exhibited by different bacterial HtrA proteins in bacteria under selective pressure, and review the significance of recently developed small molecule inhibitors targeting HtrA in pathogenic bacteria.

The challenges and prospects of Escherichia coli as an organic acid production host under acid stress

OBJECTIVE

Organic acids have a wide range of applications and have attracted the attention of many industries, and their large-scale applications have led fermentation production to low-cost development. Among them, the microbial fermentation method, especially using Escherichia coli as the production host, has the advantages of fast growth and low energy consumption, and has gradually shown better advantages and prospects in organic acid fermentation production.

IMPORTANCE

However, when the opportunity comes, the acidified environment caused by the acid products accumulated during the fermentation process also challenges E. coli. The acid sensitivity of E. coli is a core problem that needs to be solved urgently. The addition of neutralizers in traditional operations led to the emergence of osmotic stress inadvertently, the addition of strong acid substances to recover products in the salt state not only increases production costs, but the discharged sewage is also harmful to the environment.

ELABORATION

This article summarizes the current status of the application of E. coli in the production of organic acids, and based on the impact of acid stress on the physiological state of cells and the impact of industrial production profits, put forward some new conjectures that can make up for the deficiencies in existing research and application.

IMPLICATION

At this point, the diversified transformation of E. coli has become a chassis microbe that is more suitable for industrial fermentation, enhancing industrial application value.

KEY POINTS

• E. coli is a potential host for high value-added organic acids production. • Classify the damage mechanism and coping strategies of E. coli when stimulated by acid molecules. • Multi-dimensional expansion tools are needed to create acid-resistant E. coli chassis.

Cell envelope proteases and peptidases of Pseudomonas aeruginosa: multiple roles, multiple mechanisms

Pseudomonas aeruginosa is a Gram-negative bacterium that is commonly isolated from damp environments. It is also a major opportunistic pathogen, causing a wide range of problematic infections. The cell envelope of P. aeruginosa, comprising the cytoplasmic membrane, periplasmic space, peptidoglycan layer and outer membrane, is critical to the bacteria's ability to adapt and thrive in a wide range of environments. Over 40 proteases and peptidases are located in the P. aeruginosa cell envelope. These enzymes play many crucial roles. They are required for protein secretion out of the cytoplasm to the periplasm, outer membrane, cell surface or the environment; for protein quality control and removal of misfolded proteins; for controlling gene expression, allowing adaptation to environmental changes; for modification and remodelling of peptidoglycan; and for metabolism of small molecules. The key roles of cell envelope proteases in ensuring normal cell functioning have prompted the development of inhibitors targeting some of these enzymes as potential new anti-Pseudomonas therapies. In this review, we summarise the current state of knowledge across the breadth of P. aeruginosa cell envelope proteases and peptidases, with an emphasis on recent findings, and highlight likely future directions in their study.

Activity-Based Probes for the High Temperature Requirement A Serine Proteases

The high temperature requirement A (HTRA) family of serine proteases mediates protein quality control. These proteins process misfolded proteins in several diseases including Alzheimer's disease (AD) and Parkinson's disease (PD). While their structures and activation mechanisms have been studied, the precise details of the regulation of their activity under physiological conditions have not been completely elucidated, partly due to the lack of suitable chemical probes. In the present study, we developed novel activity-based probes (ABPs) targeting the HTRAs and demonstrated their utility in the monitoring and quantification of changes in enzyme activity in live cells. Using our probes, we found the activity of HTRA1 to be highly elevated in an AD-like cell-based model. We also observed the active HTRA2 in live cells by using a mitochondrion-targeted probe. We believe that our probes can serve as a useful tool to study the role of human HTRAs in neurodegenerative diseases.

Bifunctional Non-Canonical Amino Acids: Combining Photo-Crosslinking with Click Chemistry

Genetic code expansion is a powerful tool for the study of protein interactions, as it allows for the site-specific incorporation of a photoreactive group via non-canonical amino acids. Recently, several groups have published bifunctional amino acids that carry a handle for click chemistry in addition to the photo-crosslinker. This allows for the specific labeling of crosslinked proteins and therefore the pulldown of peptides for further analysis. This review describes the properties and advantages of different bifunctional amino acids, and gives an overview about current and future applications.

Degp degrades a wide range of substrate proteins in Escherichia coli under stress conditions

DegP, a periplasmic dual-functional protease and chaperone in Gram-negative bacteria, is critical for bacterial stress resistance, but the precise underlying mechanisms are not fully understood. Here, we show that the protease function of DegP is critical for Escherichia coli cells to maintain membrane integrity, particularly under heat shock conditions (42°C). Site-directed photo-cross-linking, mass spectrometry and immunoblotting analyses reveal that both periplasmic proteins (e.g. OppA and MalE) and β-barrel outer membrane proteins (OMPs) are DegP-interacting proteins and that OppA is degraded by DegP in vitro and in vivo at 42°C. In addition, OmpA and BamA, chimeric β-barrel OMPs containing a soluble periplasmic domain, are bound to DegP in both unfolded and folded forms, whereas only the unfolded forms are degradable by DegP. The presence of folded OmpA as a substrate of DegP is attributed to its periplasmic domain, which is resistant to DegP degradation and even generally protects pure β-barrel OMPs from degradation in an intra-molecular way. Furthermore, a pair of residues (R262 and V328) in the PDZ domain-1 of DegP play important roles for binding unfolded and folded β-barrel OMPs, with R262 being critical. Our study, together with earlier reports, indicates that DegP plays a critical role in protein quality control in the bacterial periplasm by degrading both periplasmic proteins and β-barrel OMPs under stress conditions and likely also by participating in the folding of chimeric β-barrel OMPs. A working model is proposed to illustrate the finely tuned functions of DegP with respect to different substrate proteins.

Biogenesis, quality control, and structural dynamics of proteins as explored in living cells via site-directed photocrosslinking

Protein biogenesis and quality control are essential to maintaining a functional pool of proteins and involve numerous protein factors that dynamically and transiently interact with each other and with the substrate proteins in living cells. Conventional methods are hardly effective for studying dynamic, transient, and weak protein-protein interactions that occur in cells. Herein, we review how the site-directed photocrosslinking approach, which relies on the genetic incorporation of a photoreactive unnatural amino acid into a protein of interest at selected individual amino acid residue positions and the covalent trapping of the interacting proteins upon ultraviolent irradiation, has become a highly efficient way to explore the aspects of protein contacts in living cells. For example, in the past decade, this approach has allowed the profiling of the in vivo substrate proteins of chaperones or proteases under both physiologically optimal and stressful (e.g., acidic) conditions, mapping residues located at protein interfaces, identifying new protein factors involved in the biogenesis of membrane proteins, trapping transiently formed protein complexes, and snapshotting different structural states of a protein. We anticipate that the site-directed photocrosslinking approach will play a fundamental role in dissecting the detailed mechanisms of protein biogenesis, quality control, and dynamics in the future.