STRUCTURAL BASIS OF SUGAR TRANSLOCATION THROUGH MALTOPORIN CHANNELS. T. Schirmer, R. Dutzler, Y.-F. Wang^*, J.P. Rosenbusch^* Departments of Structural Biology and ^*Microbiology, Biozentrum, University of Basel, CH-4056 Basel, Switzerland.

Maltoporin (LamB), an integral membrane protein from E. coli, facilitates the diffusion of maltooligosaccharides across the outer membrane. The structure exhibits a trimer of 18-stranded antiparallel [[beta]]-barrels. Each barrel contains a channel with an exposed hydrophobic patch formed by six aromatic residues ('greasy slide') that are linearly arranged. Apart from this the channel is lined exclusively by ionizable residues.

Crystal structures of maltoporin in complex with maltooligosaccharides of various lengths reveal that the sugars are bound to the channel constriction and are in apolar contact with the 'greasy slide'. A multitude of H-bonds that are formed between the sugar hydroxyl groups and residues from the channel lining explain affinity and specificity of this interaction. A complex structure with sucrose reveals non-productive binding above the channel constriction. The structural data will be discussed with respect to function and a detailled path for sugar translocation will be proposed.

Schirmer, T., Keller, T. A., Wang, Y.-F. & Rosenbusch, J. P. (1995). Structural basis for sugar translocation through maltoporin channels at 3.1Å resolution. Science, 267, 512-514.

Dutzler, R., Wang, Y.-F., Rosenbusch, J. P. & Schirmer, T. (1996). Crystal structures of various maltooligosaccharides bound to maltoporin reveal a specific sugar translocation pathway. Structure, 4, 127-134.