1.B.10 The Nucleoside-specific Channel-forming Outer Membrane Porin (Tsx) Family

Functionally characterized proteins of the Tsx family are closely related proteins from enteric bacteria and their close relatives specific for nucleosides. They are of about 270 amino acyl residues in length and form monomeric 12-stranded β-barrel porins. They transport ribo- and deoxyribonucleosides as well as the antibiotic, abicidin, and they serve as receptors for bacteriophage and colicins. They possess a distant homologue in Vibrio parahaemolyticus (OmpK) which also serves as a phage receptor.

The crystal structure of Tsx of E. coli is known ± nucleoside (Ye and van den Berg, 2004). The monomeric 12-stranded β-barrel has a long narrow channel spanning the outer membrane. The channel is shaped like a keyhole and contains two well-defined nucleoside binding sites. The base moiety of the nucleoside is located in the narrow part of the keyhole, while the sugar occupies the wider opening. Pairs of aromatic residues and flanking ionizable residues are involved in nucleoside binding. Nucleoside transport presumably occurs by diffusion from one binding site to the next.

The generalized transport reaction catalyzed by charaterized members of the Tsx family is:

Nucleoside (out) Nucleoside (periplasm).