The potassium channel KcsA and its interaction with the lipid bilayer.

The crystal structure of the K+ channel KcsA explains many features of ion channel function. The selectivity filter corresponds to a narrow region about 12 Along and 3 A wide, lined by carbonyl groups of the peptide backbone, through which a K+ ion can only move ina dehydrated form. The selectivity filter opens into a central, water-filled cavity leading to a gating site on the intracellular side of the channel. The channel is tetrameric, each monomer containing two transmembrane a helices, M1 and M2. Helix M1 faces the lipid bi-layer and helix M2 faces the central channel pore; the M2 helices participate in subunit-subunit interactions. Helices M1 and M2 in each subunit pack as a pair of antiparallel coils with a heptad repeat, but the M2 helices of neighbouring subunits show fewer interactions, crossing at an angle of about -40 degrees. Trp residues at the ends of the transmembrane a helices form clear girdles on the two faces of the membrane, which, together with girdles of charged residues, define a hydrophobic thickness of about 37 A for the channel. Binding constants for phosphatidylcholines to KcsA vary with fatty acyl chain length, the optimum chain length being C22. A phosphatidylcholine with this chain length gives a bilayer of thickness about 34 A in the liquid crystalline phase, matching the hydrophobic thickness of the protein. However, a typical biological membrane has a hydrophobic thickness of about 27 A. Thus either the transmembrane a helices of KcsA are more tilted in the native membrane than they are in the crystal structure, or the channel is under stress in the native membrane. The efficiency of hydrophobic matching between KcsA and the surrounding lipid bilayer is high over the chain length range C10-C24. The channel requires the presence of some anionic lipids for function, and fluorescence quenching studies show the presence of two classes of lipid binding site on KcsA; at one class of site (nonannular sites) anionic phospholipids bind more strongly than phosphatidylcholine, whereas at the other class of site (annular sites) phosphatidylcholines and anionic phospholipids bind with equal affinity.

Manometric rhinometry: a new method of measuring the volume of the air in the nasal cavity.

Almost all the pathological and physiological process that effect the nose will change the volume of the airspace within it. This volume has previously been difficult to measure but a new method that calculates this space has been developed. A mechanical model was built to test the physical parameters involved in making volume measurements. The model demonstrated that a model sinus could be detected if the ostium was only 0.5 mm in diameter. It also showed that a mathematical model which described the volume of the space could be constructed. In vivo experiments showed that nasal volume can be measured in children as young as 4. In children, nasal volume correlates with age, height, and weight. They had a low coefficient of variation (7.1%) and a high test-retest correlation (r = 0.94). Adult nasal volume averaged 138 ml. The method is sensitive enough to detect the decongestant effect of xylometazoline in a group of 17 healthy volunteers (p < 0.01). There is no significant difference in the sensitivity to detect the decongestant effect of xylometazoline when compared with active anterior rhinometry, nasal peak flow, and acoustic rhinometry (range 80-95%).

The calcium pumps of plant cell membranes.

Active calcium transport in higher plant cell membranes involves both H(+)-linked antiport (at the tonoplast) and direct (P-type) calcium pumping ATPases. Both systems act to remove calcium from the cytoplasm either by pumping it into intracellular stores or into the apoplast. This chapter considers recent advances in our knowledge of the calcium-pumping ATPases of the plant cell, located both at the plasma membrane and in intracellular membranes. Progress in characterising the types of Ca2+ pump in plant cells is particularly important as it becomes increasingly clear that designations applicable to other eukaryotic Ca2+ pumps ('PM-type' and 'SR/ER type') are much less relevant for plant cells. Responses of plant Ca2+ pumps to mammalian Ca2+ pump inhibitors and differences in estimated relative molecular mass also underline the differences between plant and animal Ca2+ pumps. Molecular cloning has resulted in the identification of an SR/ER type Ca2+ pump in plants strongly homologous to that of mammals. These advances are put into the context of research aims in characterising the function and mechanisms of the plant Ca2+ pumps, and their role not only in regulating cytosolic free calcium concentrations, but also in providing intracellular signalling pools and in the regulation of secretion is discussed.