LamB (maltoporin) of Salmonella typhimurium: isolation, purification and comparison of sugar binding with LamB of Escherichia coli

Authors


  • The authors would like to thank Maurice Hofnung, Katharina Bauer and Angela Schmid for many helpful discussions, and Inge Rohm, Barbara Lang and Karl-Heinz Klotz for expert technical assistance. This work was supported by the Deutsche Forschungsgemeinschaft (SFB 176) and the Fond der Chemischen Industrie.

Summary

LamB (maltporin) of Salmonella typhimurium was found to be more strongly associated with the murein than OmpF. It was purified in one step using a hydroxyapatite (HTP) column. Reconstitution of the pure protein with lipid bilayer membrane showed that LamB of S. typhimurium formed small ion-permeable channels with a single channel conductance of about 90 pS in 1 M KCI and some preference for cations over anions. The conductance concentration curve was linear, which suggested that LamB of S. typhimurium does not contain any binding site for ions. Pore conductance was completely inhibited by the addition of 20mM maltotriose. Titration of the LamB-induced membrane conductance with different sugars, including all members of the maltooligosaccharide series up to seven glucose residues, suggested that the channel contains, like LamB (maltoporin) of Escherichia coli, a binding site for sugars. The binding constant of sugars of the maltooligosaccharide series increased with increasing number of glucose residues up to five (saturated). Small sugars had a higher stability constant for sugar binding relative to LamB of E. coli. The advantage of a binding site inside a specific porin for the permeation of solutes is discussed with respect to the properties of a general diffusion porin.

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