STRUCTURAL BASIS FOR SUGAR TRANSLOCATION THROUGH MALTOPORIN CHANNELS AT 3.1-ANGSTROM RESOLUTION

Trimeric maltoporin (LamB protein) facilitates the diffusion of maltod extrins across the outer membrane of Gram-negative bacteria. The cryst al structure of maltoporin from Escherichia coli, determined to a reso lution of 3.1 angstroms, reveals an 18-stranded, antiparallel beta-bar rel that forms the framework of the channel. Three inwardly folded loo ps contribute to a constriction about halfway through the channel. Six contingent aromatic residues line the channel and form a path from th e vestibule to the periplasmic outlet. Soaking of a crystal with malto triose revealed binding of the sugar to this hydrophobic track across the constriction, which suggests that maltose and linear oligosacchari des may be translocated across the membrane by guided diffusion along this path.