Proteins of the Tsx family are nucleoside-specific receptors. Their roles vary from ribonucleosides and deoxyribonucleosides transport to receptors for bacteriophages and colicins. The crystal structure of Tsx of Escherchia coli revealed a monomeric 12-stranded ß-barrel with a long narrow channel spanning the outer membrane. The structure provides a mechanism for nucleoside transport across the bacterial outer membrane. The channel which is shaped like a keyhole has two distinct and well defined nucleoside-binding sites. Nucleoside transport presumably occurs by diffusion from one binding site to the next.

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