Frontiers in Bioscience-Landmark (FBL) is published by IMR Press from Volume 26 Issue 5 (2021). Previous articles were published by another publisher on a subscription basis, and they are hosted by IMR Press on imrpress.com as a courtesy and upon agreement with Frontiers in Bioscience.
Academic Editor: Phillip Klebba
The trimeric protein LamB of E. coli K12 (maltoporin) specifically facilitates the diffusion of maltose and maltooligosaccharides through the outer membrane and acts as a non-specific porin for small hydrophilic molecules. LamB serves also as a specific cell surface receptor for phages, including phage lambda. Each monomer consists of an eighteen-stranded antiparallel beta-barrel with nine surface loops (L1 to L9). Three loops fold into the beta-barrel, with loop L3 constricting the channel about half way. Monomers bind sugars independently of each other. Structural studies of maltoporin in complex with maltodextrin showed that the binding site, located at the channel constriction, was composed of : i) a "greasy slide", a left-handed helical arrangement of aromatic residues extending along the channel providing a hydrophobic path to the glycosyl moieties; and ii) an "ionic track", found on both sides of the channel constriction zone, providing residues available for forming hydrogen bonds with the sugars. The participation of the surface loops that cover the entry of the pore to phage binding and to sugar binding and transport has also been thoroughly investigated. Genetic and biochemical analyses suggest that some of the surface loops participate directly in the orientation and entry of maltooligosaccharides into the channel and, thus, control access to the binding site. This non-exhaustive review will summarize briefly the major methodological and conceptual advances on the analysis of LamB, during the past two decades. The role of different domains of the protein that participate to sugar transport will be described; particular emphasis will be given to the role of the surface loops. Then, we will discuss how these domains may interact functionally and propose a comprehensive model of maltodextrin transport through the LamB pore.