A Specific interaction between SecA2 and a region of the preprotein adjacent to the signal peptide occurs during transport via the accessory Sec system

The Two Distinct Types of SecA2-Dependent Export Systems

In addition to SecA of the general Sec system, many Gram-positive bacteria, including mycobacteria, express SecA2, a second, transport-associated ATPase. SecA2s can be subdivided into two mechanistically distinct types: (i) SecA2s that are part of the accessory Sec (aSec) system, a specialized transporter mediating the export of a family of serine-rich repeat (SRR) glycoproteins that function as adhesins, and (ii) SecA2s that are part of multisubstrate systems, in which SecA2 interacts with components of the general Sec system, specifically the SecYEG channel, to export multiple types of substrates. Found mainly in streptococci and staphylococci, the aSec system also contains SecY2 and novel accessory Sec proteins (Asps) that are required for optimal export. Asp2 also acetylates glucosamine residues on the SRR domains of the substrate during transport. Targeting of the SRR substrate to SecA2 and the aSec translocon is mediated by a specialized signal peptide. Multisubstrate SecA2 systems are present in mycobacteria, corynebacteria, listeriae, clostridia, and some bacillus species. Although most substrates for this SecA2 have canonical signal peptides that are required for export, targeting to SecA2 appears to depend on structural features of the mature protein. The feature of the mature domains of these proteins that renders them dependent on SecA2 for export may be their potential to fold in the cytoplasm. The discovery of aSec and multisubstrate SecA2 systems expands our appreciation of the diversity of bacterial export pathways. Here we present our current understanding of the mechanisms of each of these SecA2 systems.

Recognition of specific sialoglycan structures by oral streptococci impacts the severity of endocardial infection

Streptococcus gordonii and Streptococcus sanguinis are primary colonizers of the tooth surface. Although generally non-pathogenic in the oral environment, they are a frequent cause of infective endocarditis. Both streptococcal species express a serine-rich repeat surface adhesin that mediates attachment to sialylated glycans on mucin-like glycoproteins, but the specific sialoglycan structures recognized can vary from strain to strain. Previous studies have shown that sialoglycan binding is clearly important for aortic valve infections caused by some S. gordonii, but this process did not contribute to the virulence of a strain of S. sanguinis. However, these streptococci can bind to different subsets of sialoglycan structures. Here we generated isogenic strains of S. gordonii that differ only in the type and range of sialoglycan structures to which they adhere and examined whether this rendered them more or less virulent in a rat model of endocarditis. The findings indicate that the recognition of specific sialoglycans can either enhance or diminish pathogenicity. Binding to sialyllactosamine reduces the initial colonization of mechanically-damaged aortic valves, whereas binding to the closely-related trisaccharide sialyl T-antigen promotes higher bacterial densities in valve tissue 72 hours later. A surprising finding was that the initial attachment of streptococci to aortic valves was inversely proportional to the affinity of each strain for platelets, suggesting that binding to platelets circulating in the blood may divert bacteria away from the endocardial surface. Importantly, we found that human and rat platelet GPIbα (the major receptor for S. gordonii and S. sanguinis on platelets) display similar O-glycan structures, comprised mainly of a di-sialylated core 2 hexasaccharide, although the rat GPIbα has a more heterogenous composition of modified sialic acids. The combined results suggest that streptococcal interaction with a minor O-glycan on GPIbα may be more important than the over-all affinity for GPIbα for pathogenic effects.

Infective endocarditis (IE) is a life-threatening infection of heart valves, and streptococci that normally reside in the mouth are a leading cause of this disease. Some oral streptococcal species express a protein on their surface that enables attachment to glycan (sugar) modifications on saliva proteins, an interaction that may be important for colonization of the tooth and other oral surfaces. These "Siglec-like adhesins" are hypervariable in the type and number of glycan structures they bind, ranging from just one to more than six of the structures displayed on the saliva proteins. If streptococci enter into the bloodstream, the Siglec-like adhesin can mediate attachment to similar glycans that decorate platelet or plasma proteins, which can impact the overall virulence of the organism. This study highlights how recognition of a specific type of glycan structure can cause a generally beneficial or neutral microbe to create damage to specific tissues—in this case the heart valves, illustrating one means by which commensal bacteria can become opportunistic or accidental pathogens. The findings further indicate that certain glycan-binding streptococci among the oral microbiota may be predisposed to produce infective endocarditis.

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.

A photo-cross-linking approach to monitor protein dynamics in living cells

BACKGROUND

Proteins, which comprise one of the major classes of biomolecules that constitute a cell, interact with other cellular factors during both their biogenesis and functional states. Studying not only static but also transient interactions of proteins is important to understand their physiological roles and regulation mechanisms. However, only a limited number of methods are available to analyze the dynamic behaviors of proteins at the molecular level in a living cell. The site-directed in vivo photo-cross-linking approach is an elegant technique to capture protein interactions with high spatial resolution in a living cell.

SCOPE OF REVIEW

Here, we review the in vivo photo-cross-linking approach including its recent applications and the potential problems to be considered. We also introduce a new in vivo photo-cross-linking-based technique (PiXie) to study protein dynamics with high spatiotemporal resolution.

MAJOR CONCLUSIONS

In vivo photo-cross-linking enables us to capture weak/transient protein interactions with high spatial resolution, and allows for identification of interacting factors. Moreover, the PiXie approach can be used to monitor rapid folding/assembly processes of proteins in living cells.

GENERAL SIGNIFICANCE

In vivo photo-cross-linking is a simple method that has been used to analyze the dynamic interactions of many cellular proteins. Originally developed in Escherichia coli, this system has been extended to studies in various organisms, making it a fundamental technique for investigating dynamic protein interactions in many cellular processes. This article is part of a Special issue entitled "Novel major techniques for visualizing 'live' protein molecules" edited by Dr. Daisuke Kohda.

Membrane trafficking of the bacterial adhesin GspB and the accessory Sec transport machinery

The serine-rich repeat (SRR) glycoproteins of Gram-positive bacteria are large, cell wall-anchored adhesins that mediate binding to many host cells and proteins and are associated with bacterial virulence. SRR glycoproteins are exported to the cell surface by the accessory Sec (aSec) system comprising SecA2, SecY2, and 3-5 additional proteins (Asp1 to Asp5) that are required for substrate export. These adhesins typically have a 90-amino acid-long signal peptide containing an elongated N-region and a hydrophobic core. Previous studies of GspB (the SRR adhesin of Streptococcus gordonii) have shown that a glycine-rich motif in its hydrophobic core is essential for selective, aSec-mediated transport. However, the role of this extended N-region in transport is poorly understood. Here, using protein-lipid co-flotation assays and site-directed mutagenesis, we report that the N-region of the GspB signal peptide interacts with anionic lipids through electrostatic forces and that this interaction is necessary for GspB preprotein trafficking to lipid membranes. Moreover, we observed that protein-lipid binding is required for engagement of GspB with SecA2 and for aSec-mediated transport. We further found that SecA2 and Asp1 to Asp3 also localize selectively to liposomes that contain anionic lipids. These findings suggest that the GspB signal peptide electrostatically binds anionic lipids at the cell membrane, where it encounters SecA2. After SecA2 engagement with the signal peptide, Asp1 to Asp3 promote SecA2 engagement with the mature domain, which activates GspB translocation.

O-acetylation of the serine-rich repeat glycoprotein GspB is coordinated with accessory Sec transport

The serine-rich repeat (SRR) glycoproteins are a family of adhesins found in many Gram-positive bacteria. Expression of the SRR adhesins has been linked to virulence for a variety of infections, including streptococcal endocarditis. The SRR preproteins undergo intracellular glycosylation, followed by export via the accessory Sec (aSec) system. This specialized transporter is comprised of SecA2, SecY2 and three to five accessory Sec proteins (Asps) that are required for export. Although the post-translational modification and transport of the SRR adhesins have been viewed as distinct processes, we found that Asp2 of Streptococcus gordonii also has an important role in modifying the SRR adhesin GspB. Biochemical analysis and mass spectrometry indicate that Asp2 is an acetyltransferase that modifies N-acetylglucosamine (GlcNAc) moieties on the SRR domains of GspB. Targeted mutations of the predicted Asp2 catalytic domain had no effect on transport, but abolished acetylation. Acetylated forms of GspB were only detected when the protein was exported via the aSec system, but not when transport was abolished by secA2 deletion. In addition, GspB variants rerouted to export via the canonical Sec pathway also lacked O-acetylation, demonstrating that this modification is specific to export via the aSec system. Streptococci expressing GspB lacking O-acetylated GlcNAc were significantly reduced in their ability bind to human platelets in vitro, an interaction that has been strongly linked to virulence in the setting of endocarditis. These results demonstrate that Asp2 is a bifunctional protein involved in both the post-translational modification and transport of SRR glycoproteins. In addition, these findings indicate that these processes are coordinated during the biogenesis of SRR glycoproteins, such that the adhesin is optimally modified for binding. This requirement for the coupling of modification and export may explain the co-evolution of the SRR glycoproteins with their specialized glycan modifying and export systems.

Activity of a bacterial cell envelope stress response is controlled by the interaction of a protein binding domain with different partners

The bacterial phage shock protein (Psp) system is a highly conserved cell envelope stress response required for virulence in Yersinia enterocolitica and Salmonella enterica. In non-inducing conditions the transcription factor PspF is inhibited by an interaction with PspA. In contrast, PspA associates with the cytoplasmic membrane proteins PspBC during inducing conditions. This has led to the proposal that PspBC exists in an OFF state, which cannot recruit PspA, or an ON state, which can. However, nothing was known about the difference between these two states. Here, we provide evidence that it is the C-terminal domain of Y. enterocolitica PspC (PspC(CT)) that interacts directly with PspA, both in vivo and in vitro. Site-specific photocross-linking revealed that this interaction occurred only during Psp-inducing conditions in vivo. Importantly, we have also discovered that PspC(CT) can interact with the C-terminal domain of PspB (PspC(CT)·PspB(CT)). However, the PspC(CT)·PspB(CT) and PspC(CT)·PspA interactions were mutually exclusive in vitro. Furthermore, in vivo, PspC(CT) contacted PspB(CT) in the OFF state, whereas it contacted PspA in the ON state. These findings provide the first description of the previously proposed PspBC OFF and ON states and reveal that the regulatory switch is centered on a PspC(CT) partner-switching mechanism.

Illumination of growth, division and secretion by metabolic labeling of the bacterial cell surface

The cell surface is the essential interface between a bacterium and its surroundings. Composed primarily of molecules that are not directly genetically encoded, this highly dynamic structure accommodates the basic cellular processes of growth and division as well as the transport of molecules between the cytoplasm and the extracellular milieu. In this review, we describe aspects of bacterial growth, division and secretion that have recently been uncovered by metabolic labeling of the cell envelope. Metabolite derivatives can be used to label a variety of macromolecules, from proteins to non-genetically-encoded glycans and lipids. The embedded metabolite enables precise tracking in time and space, and the versatility of newer chemoselective detection methods offers the ability to execute multiple experiments concurrently. In addition to reviewing the discoveries enabled by metabolic labeling of the bacterial cell envelope, we also discuss the potential of these techniques for translational applications. Finally, we offer some guidelines for implementing this emerging technology.

Selective transport by SecA2: an expanding family of customized motor proteins

The SecA2 proteins are a special class of transport-associated ATPases that are related to the SecA component of the general Sec system, and are found in an increasingly large number of Gram-positive bacterial species. The SecA2 substrates are typically linked to the cell wall, but may be lipid-linked, peptidoglycan-linked, or non-covalently associated S-layer proteins. These substrates can have a significant impact on virulence of pathogenic organisms, but may also aid colonization by commensals. The SecA2 orthologues range from being highly similar to their SecA paralogues, to being distinctly different in apparent structure and function. Two broad classes of SecA2 are evident. One transports multiple substrates, and may interact with the general Sec system, or with an as yet unidentified transmembrane channel. The second type transports a single substrate, and is a component of the accessory Sec system, which includes the SecY paralogue SecY2 along with the accessory Sec proteins Asp1-3. Recent studies indicate that the latter three proteins may have a unique role in coordinating post-translational modification of the substrate with transport by SecA2. Comparative functional and phylogenetic analyses suggest that each SecA2 may be uniquely adapted for a specific type of substrate. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Leaving home ain't easy: protein export systems in Gram-positive bacteria

Transport of proteins into or across biological membranes is catalyzed by membrane-bound transport machineries. In Gram-positive bacteria, the vast majority of proteins are exported out of the cytosol by the conserved general secretion (Sec) system or, alternatively, by the twin-arginine translocation (Tat) system, that closely resemble their well-studied counterparts in Gram-negative bacteria. Besides these common major export routes, additional unique protein export systems (such as accessory Sec2 systems and/or type VII/WXG100 secretion systems) exist in some Gram-positive bacteria that are specifically involved in the secretion of limited subsets of proteins.

Protein export by the mycobacterial SecA2 system is determined by the preprotein mature domain

At the core of the bacterial general secretion (Sec) pathway is the SecA ATPase, which powers translocation of unfolded preproteins containing Sec signal sequences through the SecYEG membrane channel. Mycobacteria have two nonredundant SecA homologs: SecA1 and SecA2. While the essential SecA1 handles "housekeeping" export, the nonessential SecA2 exports a subset of proteins and is required for Mycobacterium tuberculosis virulence. Currently, it is not understood how SecA2 contributes to Sec export in mycobacteria. In this study, we focused on identifying the features of two SecA2 substrates that target them to SecA2 for export, the Ms1704 and Ms1712 lipoproteins of the model organism Mycobacterium smegmatis. We found that the mature domains of Ms1704 and Ms1712, not the N-terminal signal sequences, confer SecA2-dependent export. We also demonstrated that the lipid modification and the extreme N terminus of the mature protein do not impart the requirement for SecA2 in export. We further showed that the Ms1704 mature domain can be efficiently exported by the twin-arginine translocation (Tat) pathway. Because the Tat system exports only folded proteins, this result implies that SecA2 substrates can fold in the cytoplasm and suggests a putative role of SecA2 in enabling export of such proteins. Thus, the mycobacterial SecA2 system may represent another way that bacteria solve the problem of exporting proteins that can fold in the cytoplasm.

Differential localization of the streptococcal accessory sec components and implications for substrate export

The accessory Sec system of Streptococcus gordonii is comprised of SecY2, SecA2, and five proteins (Asp1 through -5) that are required for the export of a serine-rich glycoprotein, GspB. We have previously shown that a number of the Asps interact with GspB, SecA2, or each other. To further define the roles of these Asps in export, we examined their subcellular localization in S. gordonii and in Escherichia coli expressing the streptococcal accessory Sec system. In particular, we assessed how the locations of these accessory Sec proteins were altered by the presence of other components. Using fluorescence microscopy, we found in E. coli that SecA2 localized within multiple foci at the cell membrane, regardless of whether other accessory Sec proteins were expressed. Asp2 alone localized to the cell poles but formed a similar punctate pattern at the membrane when SecA2 was present. Asp1 and Asp3 localized diffusely in the cytosol when expressed alone or with SecA2. However, these proteins redistributed to the membrane in a punctate arrangement when all of the accessory Sec components were present. Cell fractionation studies with S. gordonii further corroborated these microscopy results. Collectively, these findings indicate that Asp1 to -3 are not integral membrane proteins that form structural parts of the translocation channel. Instead, SecA2 serves as a docking site for Asp2, which in turn attracts a complex of Asp1 and Asp3 to the membrane. These protein interactions may be important for the trafficking of GspB to the cell membrane and its subsequent translocation.

Getting across the cell envelope: mycobacterial protein secretion

Protein secretion is an essential determinant of mycobacterial virulence. Mycobacterium tuberculosis has a unique cell envelope consisting of two lipid bilayers, which requires dedicated protein secretion pathways. The conserved general Sec and Tat translocation systems are responsible for protein transport across the inner membrane and are both essential. Additionally, the accessory Sec pathway specifically contributes to virulence. How transport of Sec/Tat substrates across the outer membrane is accomplished is currently an enigma. In addition to these pathways, M. tuberculosis also developed specialized secretion systems for protein transport across both membranes, the type VII or ESX secretion systems. Here, we discuss our current knowledge about the mechanisms and substrates of these different protein translocation systems and their role in mycobacterial physiology and virulence.