ABSTRACT
Transporters are a diverse group of membrane proteins that facilitate the movement of water-soluble solutes through the lipid bilayer of biological membranes. The least complex transporters are the uniporters (Kakuda and MacLeod, 1994). They simply facilitate the diffusion of their substrates across membranes and dissipate substrate gradients that arise as the result of other processes. Uniporters fall into two classes, channels and carriers, which mediate transmembrane solute movement by fundamentally different mechanisms. Channel proteins contain hydrophilic pores that span the lipid bilayer. Water-soluble solutes move across the membrane by diffusion through these pores. The lumen of a channel is accessible from either side of the membrane simultaneously. Carriers may also span the lipid bilayer, but their substrate binding sites are never accessible from both sides of the membrane simultaneously. Because of this difference in mechanism, carriers but not channels are able to mediate countertransport (Harold, 1986). Since carriers are much more closely related, at least mechanistically, to symporters and antiporters than to channels, they are the type of uniporter emphasized in this volume.
