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.