Periplasmic Protein HdeA Exhibits Chaperone-like Activity Exclusively within Stomach pH Range by Transforming into Disordered Conformation

Abstract

The extremely acidic environment of the mammalian stomach, with a pH range usually between 1 and 3, represents a stressful challenge for enteric pathogenic bacteria such as Escherichia coli before they enter into the intestine. The hdeA gene of E. coli was found to be acid inducible and was revealed by genetic studies to be important for the acid survival of the strain. This study was performed in an attempt to characterize the mechanism of the activity of the HdeA protein. Our data provided in this report strongly suggest that HdeA employs a novel strategy to modulate its chaperone activity: it possesses an ordered conformation that is unable to bind denatured substrate proteins under normal physiological conditions (i.e. at neutral pH) and transforms into a globally disordered conformation that is able to bind substrate proteins under stress conditions (i.e. at a pH below 3). Furthermore, our data indicate that HdeA exposes hydrophobic surfaces that appear to be involved in the binding of denatured substrate proteins at extremely low pH values. In light of our observations, models are proposed to explain the action of HdeA in both a physiological and a molecular context.

Protein refolding by pH-triggered chaperone binding and release

“…In vitro studies demonstrated that at neutral pH, HdeA is a well-folded dimer with no chaperone activity (4). Upon exposure to low pH, HdeA, like many other proteins, loses structure (6,7). However, in contrast to most other proteins, which are inactivated upon pH-induced unfolding, partial unfolding and dissociation of HdeA into monomers causes the activation of its chaperone function (4,7).…”

“…Upon exposure to low pH, HdeA, like many other proteins, loses structure (6,7). However, in contrast to most other proteins, which are inactivated upon pH-induced unfolding, partial unfolding and dissociation of HdeA into monomers causes the activation of its chaperone function (4,7). We recently reported that low pH induces the exposure of structurally plastic, high-affinity binding sites for unfolding proteins on HdeA, and enables it to effectively prevent protein aggregation in vitro (8).…”

“…The partially unfolded and therefore highly flexible architecture of active HdeA monomers seems to contribute to HdeA's ability to adaptively bind to a wide range of different substrate proteins, some of which are much larger than the 9.7 kDa chaperone itself (8). Inactivation of HdeA is triggered by pH neutralization, which induces its refolding and dimerization (7).…”

Role of the Multidrug Resistance Regulator MarA in Global Regulation of the hdeAB Acid Resistance Operon in Escherichia coli

“…Both chaperones are only active below pH 3 and prevent irreversible aggregation of acid-denatured periplasmic proteins (14,21,23,28). HdeA seems to play a major role at pH 2, whereas HdeB is more active at pH 3 (23).…”

Identification of Campylobacter jejuni Genes Contributing to Acid Adaptation by Transcriptional Profiling and Genome-Wide Mutagenesis

“…Some of the best-characterized systems involve amino acid decarboxylases, which consume protons during amino acid decarboxylation, and antiporters that exchange product for substrate (29). Finally, DNA repair systems (10,81,82) and chaperones (8,31,41,46,50) appear to be important for the reparation of cellular material damaged by exposure to acid.…”