Identifying Swelling-activated Channels from Ion Selectivity Patterns

The molecular identity of the channels producing swelling-activated Cl conductance (g swell) is of great interest since this physiologic response to swelling is common to many cells. the question of whether the observed g swell represents a family of channel proteins is presently unanswered, but to be included in this family a channel must be activated by cell swelling and conduct Cl. Determining if a candidate Cl channel should be included based on swelling sensitivity may be problematic since the volume-sensing element could reside in an auxiliary protein that associates with the channel protein and, through allosteric modulation, creates a swelling-activated complex. If the associated sensor is removed inadvertently by experimental manipulation or incomplete reconstitution, then a legitimate family member would be excluded. Perme-ation selectivity is intrinsic to the pore of the channel protein and thus provides a less easily lost characteristic to aid in identification. Clearly, permeation by Cl is a characteristic of g swell , but organic osmolytes also flow through these channels at significant rates. Taurine is an important component of intracellular osmolarity in many cell types and can exit through these channels, which indicates a pore large enough for navigation by ions bulkier than Cl. Determination of pore size, to delimit the class of permeating molecules, has been a goal of ion selectivity studies; as pioneered by Hille (1971), the method involves calculating relative permeabilities (P X /P Cl) from reversal potential measurements combined with knowledge of ion dimensions (using space-filling models) and affinities. A plot of relative permeability versus ion diameter provides insight into pore geometry; P X /P Cl reported for g swell (Jackson et al., 1994, 1996) are shown in Fig. 1, together with values for CLC-1 (Rychkov et al., 1998), a member of a large Cl channel family. Anion diameter often has been estimated using Stokes' Law for spherical particles (Bormann et al., 1987; Rychkov et al., 1998); unfortunately, Stokes' diameters often severely underestimate ion size for diameters Ͻ 1.0 nm (Robinson and Stokes, 1959), which includes most an-ions and osmolytes of interest during swelling activation. For this reason, molecular sizes obtained from space-filling models are the most appropriate, which suggests that the pore sizes of the GABA and glycine receptor Cl channel family and the CLC family need to be reevaluated. The permeability patterns for g swell and CLC-1 (Fig. 1) illustrate two distinct classes of anion selectivity. For g swell …