1. Department of Physiology & Cellular Biophysics,
2. Department of Psychiatry, College of Physicians and Surgeons, Columbia University, 630 West 168th Street, New York, New York 10032, USA
3. Division of Molecular Therapeutics, New York State Psychiatric Institute, 1051 Riverside Drive, New York, New York 10032, USA

Correspondence to: Ming Zhou¹ Correspondence and requests for materials should be addressed to M.Z. (Email: mz2140@columbia.edu).

Abstract

Urea is highly concentrated in the mammalian kidney to produce the osmotic gradient necessary for water re-absorption. Free diffusion of urea across cell membranes is slow owing to its high polarity, and specialized urea transporters have evolved to achieve rapid and selective urea permeation. Here we present the 2.3 Å structure of a functional urea transporter from the bacterium Desulfovibrio vulgaris. The transporter is a homotrimer, and each subunit contains a continuous membrane-spanning pore formed by the two homologous halves of the protein. The pore contains a constricted selectivity filter that can accommodate several dehydrated urea molecules in single file. Backbone and side-chain oxygen atoms provide continuous coordination of urea as it progresses through the filter, and well-placed -helix dipoles provide further compensation for dehydration energy. These results establish that the urea transporter operates by a channel-like mechanism and reveal the physical and chemical basis of urea selectivity.