Structural basis of chaperone function and pilus biogenesis

An extended hydrophobic interactive surface of Yersinia pestis Caf1M chaperone is essential for subunit binding and F1 capsule assembly

A single polypeptide subunit, Caf1, polymerizes to form a dense, poorly defined structure (F1 capsule) on the surface of Yersinia pestis. The caf-encoded assembly components belong to the chaperone-usher protein family involved in the assembly of composite adhesive pili, but the Caf1M chaperone itself belongs to a distinct subfamily. One unique feature of this subfamily is the possession of a long, variable sequence between the F1 beta-strand and the G1 subunit binding beta-strand (FGL; F1 beta-strand to G1 beta-strand long). Deletion and insertion mutations confirmed that the FGL sequence was not essential for folding of the protein but was absolutely essential for function. Site-specific mutagenesis of individual residues identified Val-126, in particular, together with Val-128 as critical residues for the formation of a stable subunit-chaperone complex and the promotion of surface assembly. Differential effects on periplasmic polymerization of the subunit were also observed with different mutants. Together with the G1 strand, the FGL sequence has the potential to form an interactive surface of five alternating hydrophobic residues on Caf1M chaperone as well as in seven of the 10 other members of the FGL subfamily. Mutation of the absolutely conserved Arg-20 to Ser led to drastic reduction in Caf1 binding and surface assembled polymer. Thus, although Caf1M-Caf1 subunit binding almost certainly involves the basic principle of donor strand complementation elucidated for the PapD-PapK complex, a key feature unique to the chaperones of this subfamily would appear to be capping via high-affinity binding of an extended hydrophobic surface on the respective single subunits.

Components and dynamics of fiber formation define a ubiquitous biogenesis pathway for bacterial pili

Type IV pili (Tfp) are a unique class of multifunctional surface organelles in Gram-negative bacteria, which play important roles in prokaryotic cell biology. Although components of the Tfp biogenesis machinery have been characterized, it is not clear how they function or interact. Using Neisseria gonorrhoeae as a model system, we report here that organelle biogenesis can be resolved into two discrete steps: fiber formation and translocation of the fiber to the cell surface. This conclusion is based on the capturing of an intermediate state in which the organelle is retained within the cell owing to the simultaneous absence of the secretin family member and biogenesis component PilQ and the twitching motility/pilus retraction protein PilT. This finding is the first demonstration of a specific translocation defect associated with loss of secretin function, and additionally confirms the role of PilT as a conditional antagonist of stable pilus fiber formation. These findings have important implications for Tfp structure and function and are pertinent to other membrane translocation systems that utilize a highly related set of components.

Chaperone-assisted pilus assembly and bacterial attachment

Bacterial pili assembled by the chaperone-usher pathway can mediate microbial attachment, an early step in the establishment of an infection, by binding specifically to sugars present in host tissues. Recent work has begun to reveal the structural basis both of chaperone function in the biogenesis of these pili and of bacterial attachment.

Snapshots of usher-mediated protein secretion and ordered pilus assembly

Type 1 pilus biogenesis was used as a paradigm to investigate ordered macromolecular assembly at the outer cell membrane. The ability of Gram-negative bacteria to secrete proteins across their outer membrane and to assemble adhesive macromolecular structures on their surface is a defining event in pathogenesis. We elucidated genetic, biochemical, and biophysical requirements for assembly of functional type 1 pili. We discovered that the minor pilus protein FimG plays a critical role in nucleating the formation of the adhesive tip fibrillum. Genetic methods were used to trap pilus subunits during their translocation through the outer membrane usher protein, providing data on the structural interactions that occur between subunit components during type 1 pilus formation. Electron microscopic and biochemical analyses of these stepwise assembly intermediates demonstrated that translocation of pilus subunits occurs linearly through the usher's central channel, with formation of the pilus helix occurring extracellularly. Specialized pilin subunits play unique roles both in this multimerization and in the final ultrastructure of the adhesive pilus.

Multiple pathways allow protein secretion across the bacterial outer membrane

Secretion of proteins across the bacterial outer membrane takes place via a variety of mechanisms from simple one-component systems to complex multicomponent pathways. Secretion pathways can be organized into evolutionarily and functionally related groups, which highlight their relationship with organelle biogenesis. Recent work is beginning to reveal the structure and function of various secretion components and the molecular mechanisms of secretion.

Crystal structure and activity of human p23, a heat shock protein 90 co-chaperone

p23 is a co-chaperone for the heat shock protein, hsp90. This protein binds hsp90 and participates in the folding of a number of cell regulatory proteins, but its activities are still unclear. We have solved a crystal structure of human p23 lacking 35 residues at the COOH terminus. The structure reveals a disulfide-linked dimer with each subunit containing eight beta-strands in a compact antiparallel beta-sandwich fold. In solution, however, p23 is primarily monomeric and the dimer appears to be a minor component. Conserved residues are clustered on one face of the monomer and define a putative surface region and binding pocket for interaction(s) with hsp90 or protein substrates. p23 contains a COOH-terminal tail that is apparently less structured and is unresolved in the crystal structure. This tail is not needed for the binding of p23 to hsp90 or to complexes with the progesterone receptor. However, the tail is necessary for optimum active chaperoning of the progesterone receptor, as well as the passive chaperoning activity of p23 in assays measuring inhibition of heat-induced protein aggregation.

Stereoselective synthesis of optically active beta-lactams, potential inhibitors of pilus assembly in pathogenic bacteria

[reaction: see text] Optically active beta-lactams 3 are obtained in excellent yields (up to 93%) and with complete stereoselectivity from Meldrum's acid derivatives 1 and Delta(2)-thiazolines 2. A selective reduction to aldehydes 5 (R = Ar or CH(2)Ar) was then accomplished by using DIBAL-H. This rigid framework, with stereochemistry different than that of penicillin, is designed to be a suitable scaffold for the development of compounds inhibiting pilus formation in uropathogenic Escherichia coli.

PapD-like chaperones provide the missing information for folding of pilin proteins

A fundamental question in molecular biology is how proteins fold into domains that can serve as assembly modules for building up large macromolecular structures. The biogenesis of pili on the surface of Gram-negative bacteria requires the orchestration of a complex process that includes protein synthesis, folding via small chaperones, secretion, and assembly. The results presented here support the hypothesis that pilus subunit folding and biogenesis proceed via mechanisms termed donor strand complementation and donor strand exchange. Here we show that the steric information necessary for pilus subunit folding is not contained in one polypeptide sequence. Rather, the missing information is transiently donated by a strand of a small chaperone to allow folding. Providing the missing information for folding, via a 13-amino acid peptide extension to the C-terminal end of a pilus subunit, resulted in the production of a protein that no longer required the chaperone to fold. This mechanism of small periplasmic chaperone function described here deviates from classical hsp60 chaperone-assisted folding.

HYR, an extracellular module involved in cellular adhesion and related to the immunoglobulin-like fold

Domains belonging to the immunoglobulin-like fold are responsible for a wide variety of molecular recognition processes. Here we describe a new family of domains, the HYR family, which is predicted to belong to this fold, and which appears to be involved in cellular adhesion. HYR domains were identified in several eukaryotic proteins, often associated with Complement Control Protein (CCP) modules or arranged in multiple copies. Our analysis provides a sequence and structural basis for understanding the role of these domains in interaction mechanisms and leads to further characterization of heretofore undescribed repeated domains with similar folds found in several bacterial proteins involved in enzymatic activities (some chitinases) or in cell surface adhesion (streptococcal C-alpha antigen).

Secretion of virulence determinants by the general secretory pathway in gram-negative pathogens: an evolving story

Secretion of proteins by the general secretory pathway (GSP) is a two-step process requiring the Sec translocase in the inner membrane and a separate substrate-specific secretion apparatus for translocation across the outer membrane. Gram-negative bacteria with pathogenic potential use the GSP to deliver virulence factors into the extracellular environment for interaction with the host. Well-studied examples of virulence determinants using the GSP for secretion include extracellular toxins, pili, curli, autotransporters, and crystaline S-layers. This article reviews our current understanding of the GSP and discusses examples of terminal branches of the GSP which are utilized by factors implicated in bacterial virulence.

Crystal structure of Pseudomonas aeruginosa PAK pilin suggests a main-chain-dominated mode of receptor binding

Fibers of pilin monomers (pili) form the dominant adhesin of Pseudomonas aeruginosa, and they play an important role in infections by this opportunistic bacterial pathogen. Blocking adhesion is therefore a target for vaccine development. The receptor-binding site is located in a C-terminal disulphide-bonded loop of each pilin monomer, but functional binding sites are displayed only at the tip of the pilus. A factor complicating vaccination is that different bacterial strains produce distinct, and sometimes highly divergent, pilin variants. It is surprising that all strains still appear to bind a common receptor, asialo-GM1. Here, we present the 1.63 A crystal structure of pilin from P. aeruginosa strain PAK. The structure shows that the proposed receptor-binding site is formed by two beta-turns that create a surface dominated by main-chain atoms. Receptor specificity could therefore be maintained, whilst allowing side-chain variation, if the main-chain conformation is conserved. The location of the binding site relative to the proposed packing of the pilus fiber raises new issues and suggests that the current fiber model may have to be reconsidered. Finally, the structure of the C-terminal disulphide-bonded loop will provide the template for the structure-based design of a consensus sequence vaccine.

Evidence for donor strand complementation in the biogenesis of Haemophilus influenzae haemagglutinating pili

Haemophilus influenzae haemagglutinating pili are surface appendages that promote attachment to host cells and facilitate respiratory tract colonization, an essential step in the pathogenesis of disease. In contrast to other well-characterized forms of pili, H. influenzae haemagglutinating pili are two-stranded helical structures. Nevertheless, haemagglutinating pili are assembled by a pathway that involves a periplasmic chaperone and an outer membrane usher, analogous to the prototype pathway involved in the biogenesis of Escherichia coli P pili. In this study, we performed site-directed mutagenesis of the H. influenzae HifB chaperone and HifA major pilus subunit at positions homologous to sites important for chaperone-subunit interactions and subunit oligomerization in P pili. Mutations at putative subunit binding pocket residues in HifB or at the penultimate tyrosine in HifA abolished formation of HifB-HifA periplasmic complexes, whereas mutations at the -14 glycine in HifA had no effect on HifB-HifA interactions but abrogated HifA oligomerization. To define further the constraints of the interaction between HifA and HifB, we examined the interchangeability of pilus gene cluster components from H. influenzae type b strain Eagan (hifA-hifEEag) and the related H. influenzae biogroup aegyptius strain F3031 (hifA-hifEF3031). Functional pili were assembled both with HifAEag and the strain F3031 gene cluster and with HifAF3031 and the strain Eagan gene cluster, underscoring the flexibility of the H. influenzae chaperone/usher pathway in incorporating HifA subunits with significant sequence diversity. To gain additional insight into the interactive surfaces of HifA and HifB, we aligned HifA sequences from 20 different strains and then modelled the HifA structure based on the recently crystallized PapD-PapK complex. Analysis of the resulting structure revealed high levels of sequence conservation in regions predicted to interact with HifB, and maximal sequence diversity in regions potentially exposed on the surface of assembled pili. These results suggest broad applicability of structure-function relationships identified in studies of P pili, including the concepts of donor strand complementation and donor strand exchange. In addition, they provide insight into the structure of HifA and suggest a basis for antigenic variation in H. influenzae haemagglutinating pili.

Salmonella enteritidis fimbriae displaying a heterologous epitope reveal a uniquely flexible structure and assembly mechanism

Two distinct Salmonella fimbrins, AgfA and SefA, comprising thin aggregative fimbriae SEF17 and SEF14, respectively, were each genetically engineered to carry PT3, an alpha-helical 16-amino acid Leishmania T-cell epitope derived from the metalloprotease gp63. To identify regions within AgfA and SefA fimbrins amenable to replacement with this epitope, PCR-generated chimeric fimbrin genes were constructed and used to replace the native chromosomal agfA and sefA genes in Salmonella enteritidis. Immunoblot analysis using anti-SEF17 and anti-PT3 sera demonstrated that all ten AgfA chimeric fimbrin proteins were expressed by S. enteritidis under normal growth conditions. Immunoelectron microscopy confirmed that eight of the AgfA::PT3 proteins were effectively assembled into cell surface-exposed fimbriae. The PT3 replacements in AgfA altered Congo red (CR) binding, cell-cell adhesion and cell surface properties of S. enteritidis to varying degrees. However, these chimeric fimbriae were still highly stable, being resistant to proteinase K digestion and requiring harsh formic acid treatment for depolymerization. In marked contrast to AgfA, none of the chimeric SefA proteins were expressed or assembled into fimbriae. Since each PT3 replacement constituted over 10% of the AgfA amino acid sequence and all ten replacements collectively represented greater than 75% of the entire AgfA primary sequence, the ability of AgfA to accept large sequence substitutions and still assemble into fibers is unique among fimbriae and other structural proteins. This structural flexibility may be related to the novel fivefold repeating sequence of AgfA and its recently proposed structure Proper formation of chimeric fimbrial fibers suggests an unusual assembly mechanism for thin aggregative fimbriae which tolerates aberrant structures. This study opens a range of possibilities for Salmonella thin aggregative fimbriae as a carrier of heterologous epitopes and as an experimental model for studies of protein structure.

PapD-like chaperones and pilus biogenesis

The assembly of adhesive pili from individual subunits by periplasmic PapD-like chaperones in Gram-negative bacteria offers insight into the complex process of organelle biogenesis. PapD-like chaperones bind, stabilize, and cap interactive surfaces of subunits until they are assembled into the pilus. Subunits lack the seventh *gb-strand necessary to complete their immunoglobulin-like folds; the chaperone supplies this missing strand. Indeed, the chaperone may act as a template, providing steric information to facilitate subunit folding. In the mature pilus, each subunit is thought to supply the missing strand to complete the fold of its neighbor. Thus, one general function of chaperones in organelle biogenesis may be to cap highly interactive surfaces of subunits until they reach the proper assembly site.

A role for Salmonella fimbriae in intraperitoneal infections

Enteric bacteria possess multiple fimbriae, many of which play critical roles in attachment to epithelial cell surfaces. SEF14 fimbriae are only found in Salmonella enterica serovar Enteritidis (S. enteritidis) and closely related serovars, suggesting that SEF14 fimbriae may affect serovar-specific virulence traits. Despite evidence that SEF14 fimbriae are expressed by S. enteritidis in vivo, previous studies showed that SEF14 fimbriae do not mediate adhesion to the intestinal epithelium. Therefore, we tested whether SEF14 fimbriae are required for virulence at a stage in infection after the bacteria have passed the intestinal barrier. Polar mutations that disrupt the entire sef operon decreased virulence in mice more than 1,000-fold. Nonpolar mutations that disrupted sefA (encoding the major structural subunit) did not affect virulence, but mutations that disrupted sefD (encoding the putative adhesion subunit) resulted in a severe virulence defect. The results indicate that the putative SEF14 adhesion subunit is specifically required for a stage of the infection subsequent to transit across the intestinal barrier. Therefore, we tested whether SefD is required for uptake or survival in macrophages. The majority of wild-type bacteria were detected inside macrophages soon after i.p. infection, but the sefD mutants were not readily internalized by peritoneal macrophages. These results indicate that the potential SEF14 adhesion subunit is essential for efficient uptake or survival of S. enteritidis in macrophages. This report describes a role of fimbriae in intracellular infection, and indicates that fimbriae may be required for systemic infections at stages beyond the initial colonization of host epithelial surfaces.

Intramolecular chaperones: polypeptide extensions that modulate protein folding

Several prokaryotic and eukaryotic proteins are synthesized as precursors in the form of pre-pro-proteins. While the pre-regions function as signal peptides that are involved in transport, the propeptides can often catalyze correct folding of their associated proteins. Such propeptides have been termed intramolecular chaperones. In cases where propeptides may not directly catalyze the folding reaction, it appears that they can facilitate processes such as structural organization and oligomerization, localization, sorting and modulation of enzymatic activity and stability of proteins. Based on the available literature it appears that propeptides may actually function as 'post-translational modulators' of protein structure and function. Propeptides can be classified into two broad categories: Class I propeptides that function as intramolecular chaperones and directly catalyze the folding reaction; and Class II propeptides that are not directly involved in folding.

Assembly of complex organelles: pilus biogenesis in gram-negative bacteria as a model system

Pathogenic bacteria assemble a variety of adhesive structures on their surface for attachment to host cells. Some of these structures are quite complex. For example, the hair-like organelles known as pili or fimbriae are generally composed of several components and often exhibit composite morphologies. In gram-negative bacteria assembly of pili requires that the subunits cross the cytoplasmic membrane, fold correctly in the periplasm, target to the outer membrane, assemble into an ordered structure, and cross the outer membrane to the cell surface. Thus, pilus biogenesis provides a model for a number of basic biological problems including protein folding, trafficking, secretion, and the ordered assembly of proteins into complex structures. P pilus biogenesis represents one of the best-understood pilus systems. P pili are produced by 80-90% of all pyelonephritic Escherichia coli and are a major virulence determinant for urinary tract infections. Two specialized assembly factors known as the periplasmic chaperone and outer membrane usher are required for P pilus assembly. A chaperone/usher pathway is now known to be required for the biogenesis of more than 30 different adhesive structures in diverse gram-negative pathogenic bacteria. Elucidation of the chaperone/usher pathway was brought about through a powerful combination of molecular, biochemical, and biophysical techniques. This review discusses these approaches as they relate to pilus assembly, with an emphasis on newer techniques.

X-ray structure of the FimC-FimH chaperone-adhesin complex from uropathogenic Escherichia coli

Type 1 pili-adhesive fibers expressed in most members of the Enterobacteriaceae family-mediate binding to mannose receptors on host cells through the FimH adhesin. Pilus biogenesis proceeds by way of the chaperone/usher pathway. The x-ray structure of the FimC-FimH chaperone-adhesin complex from uropathogenic Escherichia coli at 2.5 angstrom resolution reveals the basis for carbohydrate recognition and for pilus assembly. The carboxyl-terminal pilin domain of FimH has an immunoglobulin-like fold, except that the seventh strand is missing, leaving part of the hydrophobic core exposed. A donor strand complementation mechanism in which the chaperone donates a strand to complete the pilin domain explains the basis for both chaperone function and pilus biogenesis.