Single-channel properties, sugar specificity, and role of chitoporin in adaptive survival of Vibrio cholerae type strain O1

Single-channel characterization of the chitooligosaccharide transporter chitoporin (SmChiP) from the opportunistic pathogen Serratia marcescens

Serratia marcescens is an opportunistic pathogen that can utilize chitin as a carbon source, through its ability to produce chitin-degrading enzymes to digest chitin and membrane transporters to transport the degradation products (chitooligosaccharides) into the cells. Further characterization of these proteins is important to understand details of chitin metabolism. Here, we investigate the properties and function of the S. marcescens chitoporin, namely SmChiP, a chitooligosaccharide transporter. We show that SmChiP is a monomeric porin that forms a stable channel in artificial phospholipid membranes, with an average single-channel conductance of 0.5 ± 0.02 nS in 1 M KCl electrolyte. Additionally, we demonstrated that SmChiP allowed the passage of small molecules with a size exclusion limit of <300 Da and exhibited substrate specificity toward chitooligosaccharides, both in membrane and detergent-solubilized forms. We found that SmChiP interacted strongly with chitopentaose (K_d = 23 ± 2.0 μM) and chitohexaose (K_d = 17 ± 0.6 μM) but did not recognize nonchitose oligosaccharides (maltohexaose and cellohexaose). Given that S. marcescens can use chitin as a primary energy source, SmChiP may serve as a target for further development of nutrient-based antimicrobial therapies directed against multidrug antibiotic-resistant S. marcescens infections.

The C2 entity of chitosugars is crucial in molecular selectivity of the Vibrio campbellii chitoporin

The marine bacterium Vibrio campbellii expresses a chitooligosaccharide-specific outer-membrane channel (chitoporin) for the efficient uptake of nutritional chitosugars that are externally produced through enzymic degradation of environmental host shell chitin. However, the principles behind the distinct substrate selectivity of chitoporins are unclear. Here, we employed black lipid membrane (BLM) electrophysiology, which handles the measurement of the flow of ionic current through porins in phospholipid bilayers for the assessment of porin conductivities, to investigate the pH dependency of chitosugar-chitoporin interactions for the bacterium's natural substrate chitohexaose and its deacetylated form, chitosan hexaose. We show that efficient passage of the N-acetylated chitohexaose through the chitoporin is facilitated by its strong affinity for the pore. In contrast, the deacetylated chitosan hexaose is impermeant; however, protonation of the C2 amino entities of chitosan hexaose allows it to be pulled through the channel in the presence of a transmembrane electric field. We concluded from this the crucial role of C2-substitution as the determining factor for chitoporin entry. A change from N-acetylamino- to amino-substitution effectively abolished the ability of approaching molecules to enter the chitoporin, with deacetylation leading to loss of the distinctive structural features of nanopore opening and pore access of chitosugars. These findings provide further understanding of the multistep pathway of chitin utilization by marine Vibrio bacteria and may guide the development of solid-state or genetically engineered biological nanopores for relevant technological applications.

A structural model for (GlcNAc)2 translocation via a periplasmic chitooligosaccharide-binding protein from marine Vibrio bacteria

VhCBP is a periplasmic chitooligosaccharide-binding protein mainly responsible for translocation of the chitooligosaccharide (GlcNAc)₂ across the double membranes of marine bacteria. However, structural and thermodynamic understanding of the sugar-binding/-release processes of VhCBP is relatively less. VhCBP displayed the greatest affinity toward (GlcNAc)₂, with lower affinity for longer-chain chitooligosaccharides [(GlcNAc)_3–4]. (GlcNAc)₄ partially occupied the closed sugar-binding groove, with two reducing-end GlcNAc units extending beyond the sugar-binding groove and barely characterized by weak electron density. Mutation of three conserved residues (Trp³⁶³, Asp³⁶⁵, and Trp⁵¹³) to Ala resulted in drastic decreases in the binding affinity toward the preferred substrate (GlcNAc)₂, indicating their significant contributions to sugar binding. The structure of the W513A–(GlcNAc)₂ complex in a ‘half-open’ conformation unveiled the intermediary step of the (GlcNAc)₂ translocation from the soluble CBP in the periplasm to the inner membrane–transporting components. Isothermal calorimetry data suggested that VhCBP adopts the high-affinity conformation to bind (GlcNAc)₂, while its low-affinity conformation facilitated sugar release. Thus, chitooligosaccharide translocation, conferred by periplasmic VhCBP, is a crucial step in the chitin catabolic pathway, allowing Vibrio bacteria to thrive in oceans where chitin is their major source of nutrients.

gbpA and chiA genes are not uniformly distributed amongst diverse Vibrio cholerae

Members of the bacterial genus Vibrio utilize chitin both as a metabolic substrate and a signal to activate natural competence. Vibrio cholerae is a bacterial enteric pathogen, sub-lineages of which can cause pandemic cholera. However, the chitin metabolic pathway in V. cholerae has been dissected using only a limited number of laboratory strains of this species. Here, we survey the complement of key chitin metabolism genes amongst 195 diverse V. cholerae. We show that the gene encoding GbpA, known to be an important colonization and virulence factor in pandemic isolates, is not ubiquitous amongst V. cholerae. We also identify a putatively novel chitinase, and present experimental evidence in support of its functionality. Our data indicate that the chitin metabolic pathway within V. cholerae is more complex than previously thought, and emphasize the importance of considering genes and functions in the context of a species in its entirety, rather than simply relying on traditional reference strains.

Probing the physiological roles of the extracellular loops of chitoporin from Vibrio campbellii

VhChiP, a sugar-specific porin found on the outer membrane of Vibrio campbellii, is responsible for the transport of chitooligosaccharides, allowing the bacterium to thrive in aquatic environments using chitin as a nutrient. We previously showed that VhChiP is composed of three identical subunits, each containing a 16-stranded β-barrel connected by eight extracellular loops and eight short periplasmic turns. This study is focused on the specific roles of three prominent extracellular loops of VhChiP-L2, L3, and L8. The deletion of L2 completely disrupted the L2-L2 interactions, thus destabilizing the protein trimers as well as the integrity of the secondary structure. The deletion of L3 caused a drastic loss in the binding affinity for sugar substrates because of the absence of a cluster of key amino acid residues that form the affinity sites. The removal of L8 induced pronounced gating, which is highly responsive to elevated potentials. Our data provide further information on the important roles of the three prominent loops of VhChiP: loop L2 maintains the trimeric structure and the integrity of secondary structure, loop L3 controls the binding affinity for sugar substrates, and loop L8 retains the stably open state of the channel.