Proteins KcsA potassium channel

Burykin, A. Kato, M. Warshel, A., Exploring the origin of the ion selectivity of the KcsA potassium channel, Proteins 2003, 52, 412-426. 

Finally, it is worth keeping in mind that the protein may be perturbed reversibly by the conditions of the NMR experiment. For example, a recent study demonstrated that removal of the bulk buffer that typically separates from the sample during magic angle spinning reversibly altered the conformation of the selectivity filter of the KcsA potassium channel addition of buffer to the rotor restored the conformation.101 Based on this observation of a hydration-induced shift in conformation, it seems prudent to maintain high hydration levels for NMR studies of proteins in general. 

Chill JH, Louis JM, Miller C, Bax A. NMR study of the tetrameric KcsA potassium channel in detergent micelles. Protein Sci. 2006 15 684-689. 

The selectivity of ion channels is often determined by only a handful of amino acids. Thus, the amino acids Glu-Glu-Glu-Glu (EEEE) determine the selectivity of the calcium channel of the heart [9] the amino acids Asp-Glu-Lys-Ala (DEKA) determine the selectivity of the sodium channel of nerve and muscle cells [10]. The structure of ion channels is notoriously difficult to determine because channel proteins do not easily crystal-hze. They are normally found in hpid membranes, and so methods suitable for crystallizing soluble proteins are not too helpful. Several structures have been determined, most notably, the KcsA potassium channel [4]. 

KcsA is a potassium channel protein from Streptomyces lividans that assembles as a tetramer.68 An X-ray diffraction study of a crystal defined the transmembrane and some extracellular portions of the structure but no structural information could be obtained for cytoplasmic domains. Since the protein assembles as a tetramer, when one spin label is present on each subunit, it is difficult to interpret the dipolar interactions because of the presence of adjacent... 

Sodium and potassium cations are often encountered in the same biological environment and the transmembrane movements of both are required as part of an enzymatic pathway as in Na+, K+-ATPase. Under these circumstances it is essential that cation-specific channels are formed. What features of the channels contribute to the selectivity Earlier the preferred geometries of Na+and K+, sixfold octahedral and eightfold cubic respectively, were proposed as the main discriminatory factors. A computational analysis by Dudev and Lim [35] has considered the effect of coordinated water, number of available coordination sites in the channel walls, and the dipoles of the coordinating groups. The researchers investigated cation complexes with valinomycin and the protein KcsA, both K+-selective, and compared these with a non-selective NaK channel. 

In this view, the 5. lividans K+ channel is a supramolecular structure in which selection and transport of K+is effected by cooperative interactions of protein, PHB and polyP. This hypothetical mechanism does not address and may not satisfy the wealth of experimental data acquired for this and other potassium channels. However, the presence in KcsA of these proven facilitators of ion selection and transport should be taken into account when resolving the manner in which the channel performs these tasks. 

Figure 1 Examples of several bacterial membrane proteins. The outer membrane (OM) of Gram-negative bacteria contains exclusively fS-barrel proteins, and three examples are shown BtuB (PDB ID 1NQF), which is the 22 p-stranded TonB-dependent active transporter for vitamin B 2/ th LamB or maltoporin trimer (PDB ID 1AF6), which is the 18 p-stranded passive sugar transporter and OmpA (PDB ID 1BXW), which is an 8 p-stranded protein that provides structural support for the OM. Proteins in the cytoplasmic membrane (CM) are helical, and three examples are shown the potassium channel KcsA (PDB ID 1BL8), which is a tetramer Sec YEG (PDB ID 1RH5), which forms the protein transport channel in Methanococcus and BtuCD (PDB ID ...

Figure 11 Structural biology of potassium channels. An illustration of the Kcsa K+ channel showing protein backbone and the pore-forming helices (top). In the central cavity a fully solvated potassium ion is observed (bottom) (reproduced with permission from Roux and MacKinnon ).

An exciting model within these protein/polyP/PHB complexes has been represented by a potassium channel of Streptomyces lividans KcsA [44], [47]. Association of the polymers appears to contribute to the ion selectivity and gating properties of KcsA, and to determine its preference between mono- and divalent... 

Figure 2 Two sub-units of the KcsA (left) and MthK (right) potassium channels embedded in an explicit POPC lipid bilayer. The atoms lining the selectivity filter are represented as spheres to show the individual cages which represent the binding sites of K+ ions. The KcsA and MthK structures are obtained from the protein database codes IblS.pdb" and llnq.pdb," respectively.

The four levels of structure are illustrated in Fig. 13.30 for the potassium channel KcsA protein [233]. It is quite remarkable that no matter how complicated the molecular structure of proteins is, they can quickly find their natural folded state once they have been synthesized. The way in which proteins fold into their stable structure when they are in their natural environment (aqueous solution of specific pH) remains one of the most intriguing and difficult unsolved problems in molecular biology and has attracted much attention in recent years. 

Figure 13.30. Illustration of the four levels of structure in the potassium channel KcsA protein. Left a small section of the protein showing the primary structure, that is, the sequence of aminoacids identified by the side chains, and the secondary structure, that is, the formation of an a-helix. Right the tertiary structure, that is, the formation of larger subunits conisting of several a helices (in this case each subunit consists of three helices), and the quaternary structure, that is, the arrangement of the three-helix subunits in a pattern with four-fold rotational symmetry around a central axis. The a-helix on the left is the longest helix in each of the subunits, which are each shown in slightly different shades. The ion channel that this protein forms is at the center of structure. (Figure provided by P.L. Maragakis based on data from Ref. [232].)...

The biocompatibility of PHAs originates from the fact that some monomers incorporated into the polymer chain occur naturally in the human body. The monomer (i )-3-hydroxybutyric acid is a normal metabohte found in human blood. This hydroxy acid is present at concentrations of 3 10 mg per 100 ml blood in healthy adults. Also low molecular weight PHAs are found com-plexed to other cellular macromolecules - hence they are called complexed PHAs (cPHAs). For example, cPHAs have been found in human tissues complexed with low-density lipoproteins, carrier protein albumin and in the potassium channel (KcsA) of Streptomyces lividans. Biocompatibility of PHAs, like any other biomaterial, is dependent on factors such as shape, surface porosity, surface hydrophilicity, surface energy, chemistry of the material and its degradation product. In tissue engineering, it is important that the... 

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