Structural and functional homology between periplasmic bacterial molecular chaperones and small heat shock proteins

Bacterial Adhesion Pili

Escherichia coli bacterial cells produce multiple types of adhesion pili that mediate cell-cell and cell-host attachments. These pili (also called 'fimbriae') are large biopolymers that are comprised of subunits assembled via a sophisticated micro-machinery into helix-like structures that are anchored in the bacterial outer membrane. They are commonly essential for initiation of disease and thus provide a potential target for antibacterial prevention and treatment. To develop new therapeutics for disease prevention and treatment we need to understand the molecular mechanisms and the direct role of adhesion pili during pathogenesis. These helix-like pilus structures possess fascinating and unique biomechanical properties that have been thoroughly investigated using high-resolution imaging techniques, force spectroscopy and fluid flow chambers. In this chapter, we first discuss the structure of pili and the micro-machinery responsible for the assembly process. Thereafter, we present methods for measurement of the biomechanics of adhesion pili, including optical tweezers. Data demonstrate unique biomechanical properties of pili that allow bacteria to sustain binding during in vivo fluid shear forces. We thereafter summarize the current biomechanical findings related to adhesion pili and show that pili biomechanical properties are niche-specific. That is, the data suggest that there is an organ-specific adaptation of pili that facilitates infection of the bacteria's target tissue. Thus, pilus biophysical properties are an important part of Escherichia coli pathogenesis, allowing bacteria to overcome hydrodynamic challenges in diverse environments.

Structural and functional significance of the FGL sequence of the periplasmic chaperone Caf1M of Yersinia pestis

The periplasmic molecular chaperone Caf1M of Yersinia pestis is a typical representative of a subfamily of specific chaperones involved in assembly of surface adhesins with a very simple structure. One characteristic feature of this Caf1M-like subfamily is possession of an extended, variable sequence (termed FGL) between the F1 and subunit binding G1 beta-strands. In contrast, FGS subfamily members, characterized by PapD, have a short F1-G1 loop and are involved in assembly of complex pili. To elucidate the structural and functional significance of the FGL sequence, a mutant Caf1M molecule (dCaf1M), in which the 27 amino acid residues between the F1 and G1 beta-strands had been deleted, was constructed. Expression of the mutated caf1M in Escherichia coli resulted in accumulation of high levels of dCaf1M. The far-UV circular dichroism spectra of the mutant and wild-type proteins were indistinguishable and exhibited practically the same temperature and pH dependencies. Thus, the FGL sequence of Caf1M clearly does not contribute significantly to the stability of the protein conformation. Preferential cleavage of Caf1M by trypsin at Lys-119 confirmed surface exposure of this part of the FGL sequence in the isolated chaperone and periplasmic chaperone-subunit complex. There was no evidence of surface-localized Caf1 subunit in the presence of the Caf1A outer membrane protein and dCaf1M. In contrast to Caf1M, dCaf1M was not able to form a stable complex with Caf1 nor could it protect the subunit from proteolytic degradation in vivo. This demonstration that the FGL sequence is required for stable chaperone-subunit interaction, but not for folding of a stable chaperone, provides a sound basis for future detailed molecular analyses of the FGL subfamily of chaperones.

alpha-Crystallin C-terminal domain: on the track of an Ig fold

New results obtained from a two-dimensional sequence analysis of the small heat shock protein (shsp) family are described. It is confirmed that the conserved C-terminal alpha-crystallin domain is essentially made of beta-strands, most probably two groups of beta-strands separated by a large loop. A direct correspondence between the putative beta-strands that have been identified in shsps and the seven beta-strands of a classical immunoglobulin-like fold is proposed. The hypothesis that the shsp family could belong to the immunoglobulin superfamily (IgSF) is consistent with the ubiquitous distribution and the multifunctional properties of the crystallins that are now emerging.

The small heat-shock protein IbpB from Escherichia coli stabilizes stress-denatured proteins for subsequent refolding by a multichaperone network

The role of small heat-shock proteins in Escherichia coli is still enigmatic. We show here that the small heat-shock protein IbpB is a molecular chaperone that assists the refolding of denatured proteins in the presence of other chaperones. IbpB oligomers bind and stabilize heat-denatured malate dehydrogenase (MDH) and urea-denatured lactate dehydrogenase and thus prevent the irreversible aggregation of these proteins during stress. While IbpB-stabilized proteins alone do not refold spontaneously, they are specifically delivered to the DnaK/DnaJ/GrpE (KJE) chaperone system where they refold in a strict ATPase-dependent manner. Although GroEL/GroES (LS) chaperonins do not interact directly with IbpB-released proteins, LS accelerate the rate of KJE-mediated refolding of IbpB-released MDH, and to a lesser extent lactate dehydrogenase, by rapidly processing KJE-released early intermediates. Kinetic and gel-filtration analysis showed that denatured MDH preferentially transfers from IbpB to KJE, then from KJE to LS, and then forms a active enzyme. IbpB thus stabilizes aggregation-prone folding intermediates during stress and, as an integral part of a cooperative multichaperone network, is involved in the active refolding of stress-denatured proteins.

Influence of the conserved disulphide bond, exposed to the putative binding pocket, on the structure and function of the immunoglobulin-like molecular chaperone Caf1M of Yersinia pestis

The Yersinia pestis protein Caf1M is a typical representative of a subfamily of periplasmic molecular chaperones with characteristic structural and functional features, one of which is the location of two conserved cysteine residues close to the putative binding pocket. We show that these residues form a disulphide bond, the reduction and alkylation of which significantly increases the dissociation constant of the Caf1M-Caf1 (where Caf 1 is a polypeptide subunit of the capsule) complex [from a Kd of (4.77+/-0.50)x10(-9) M for the intact protein to one of (3.68+/-0.68)x10(-8) M for the modified protein]. The importance of the disulphide bond for the formation of functional Caf1M in vivo was demonstrated using an Escherichia coli dsbA mutant carrying the Y. pestis f1 operon. In accordance with the CD and fluorescence measurements, the disulphide bond is not important for maintenance of the overall structure of the Caf1M molecule, but would appear to affect the fine structural properties of the subunit binding site. A three-dimensional model of the Caf1M-Caf1 complex was designed based on the published crystal structure of PapD (a chaperone required for Pap pili assembly) complexed with a peptide corresponding to the C-terminus of the papG subunit. In the model the disulphide bond is in close proximity to the invariant Caf1M Arg-23 and Lys-142 residues that are assumed to anchor the C-terminal group of the subunit. The importance of this characteristic disulphide bond for the orchestration of the binding site and subunit binding, as well as for the folding of the protein in vivo, is likely to be a common feature of this subfamily of Caf1M-like chaperones. A possible model for the role of the disulphide bond in Caf1 assembly is discussed.