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Specialized transmembrane proteins form channels that facilitate the transport of ions and other molecules across the membranes and thus play crucial role in many life processes. The understanding of these systems has increased enormously in recent years because of advances in structural and biochemical studies (e.g., refs. 1 and 2). However, key questions remain with regards to the exact factors that control the selectivity and gating of ion transport across membranes. To understand these factors, it would very useful to quantify the energy contributions that allow some ions to permeate while blocking others, and thus allow channels to fulfill their specific physiological functions. Although the availability of structural information paved the way for detailed scrutiny of different hypotheses and models, there is still a clear need for well defined experimental test cases. In this issue of PNAS, a study of aquaporin 6 (AQP6) by Liu et al. (3) provides a crucial test case in which a mutation of a single residue (a change of Asn-60 to Gly) completely changes the conductance, converting AQP6 from an anion channel to a water-selective channel. This finding provides extremely valuable information whose full “translation” to a functional model probably will require more structural information as well as the use of computer modeling approaches. This is not the first time that studies of the aqauporin family have provided a challenge and a guide to the understanding of ion selectivity. Most members of the aquaporin family allow water molecules to pass through membranes but block ions, including hydronium ions (2). The reason for the blockage of proton transport through aquaporin was the subject of recent studies (4–8 …

Inverting the selectivity of aquaporin 6: Gating versus direct electrostatic interaction