Ion selectivity is one of the basic properties that define an ion channel. Most tetrameric cation channels, which include the K+, Ca2+, Na+ and cyclic nucleotide-gated channels, probably share a similar overall architecture in their ion-conduction pore, but the structural details that determine ion selection are different. Although K+ channel selectivity has been well studied from a structural perspective, little is known about the structure of other cation channels. Here we present crystal structures of the NaK channel from Bacillus cereus, a non-selective tetrameric cation channel, in its Na+- and K+-bound states at 2.4 A and 2.8 A resolution, respectively. The NaK channel shares high sequence homology and a similar overall structure with the bacterial KcsA K+ channel, but its selectivity filter adopts a different architecture. Unlike a K+ channel selectivity filter, which contains four equivalent K+-binding sites, the selectivity filter of the NaK channel preserves the two cation-binding sites equivalent to sites 3 and 4 of a K+ channel, whereas the region corresponding to sites 1 and 2 of a K+ channel becomes a vestibule in which ions can diffuse but not bind specifically. Functional analysis using an 86Rb flux assay shows that the NaK channel can conduct both Na+ and K+ ions. We conclude that the sequence of the NaK selectivity filter resembles that of a cyclic nucleotide-gated channel and its structure may represent that of a cyclic nucleotide-gated channel pore.

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Selective ion conduction across ion channel pores is central to cellular physiology. To understand the underlying principles of ion selectivity in tetrameric cation channels, we engineered a set of cation channel pores based on the nonselective NaK channel and determined their structures to high resolution. These structures showcase an ensemble of selectivity filters with a various number of contiguous ion binding sites ranging from 2 to 4, with each individual site maintaining a geometry and ligand environment virtually identical to that of equivalent sites in K þ channel selectivity filters. Combined with single channel electrophysiology, we show that only the channel with four ion binding sites is K þ selective, whereas those with two or three are nonselective and permeate Na þ and K þ equally well. These observations strongly suggest that the number of contiguous ion binding sites in a single file is the key determinant of the channel's selectivity properties and the presence of four sites in K þ channels is essential for highly selective and efficient permeation of K þ ions.nonselective cation channel | potassium channel T he structure determination of several K þ selective and, more recently, bacterial nonselective cation channels has, over the past decade, vastly increased our knowledge of ion selectivity mechanisms (1-8). In K þ channels, the TVGYG signature sequence has emerged as the fundamental element imparting high K þ over Na þ selectivity (9, 10), forming four contiguous and chemically equivalent K þ binding sites composed of backbone carbonyl oxygen atoms from filter residues together with the hydroxyl oxygen atoms from the conserved threonine. Conversely, the TVGDG filter sequence of the NaK channel from Bacillus cereus, although similar in length and amino acid composition to those of K þ channels, forms a nonselective filter preserving the two most intracellular sites of K þ channel filters along with a wide, water-filled vestibular region on the immediately extracellular side (7) (Fig. 1A). Debate lingers in the field about the underlying factors contributing to these selectivity differences. Although available structural studies of K þ channels seem to favor the classical snug-fit model to account for K þ over Na þ selectivity (11, 12), computational studies on K þ channel selectivity, in some cases using an isolated K þ binding site in their calculations, usually invoke one or more of the following concepts: the coordination number of the ion, chemistry of carbonyl oxygen ligands, intrinsic dynamism of the selectivity filter, solvent exposure of the ion binding sites, or the free energy landscapes of ion entrance and translocation in a multiion configuration (13-21). To further understand the underlying principles of ion selectivity in tetrameric cation channels, we engineered a set of cation channel pores whose selectivity filters contain three (equivalent to sites 2-4 or sites 1-3 of a K þ channel) or four (equivalent to sites 1-4 of a K þ channel) ion binding sites and determined their s...

A high 5-hydroxymethylfurfural (HMF) yield of 53 mol% was obtained by direct degradation of cellulose in a biphasic system with concentrated NaHSO 4 and ZnSO 4 as co-catalysts, with 96% of cellulose conversion in 60 min. The high concentration of catalysts in the aqueous solution and the high volume ratio of organic phase to aqueous phase were responsible for the excellent performance. The depolymerization of cellulose is the rate-determine step, and the formed glucose could be efficiently converted by concentrated catalysts in the aqueous solution, leading to low concentration of glucose in the solution and thus suppressing the side reactions such as humin and char formation. † Electronic supplementary information (ESI) available. See