Workshop on Ion Channels, Mon, 08 Oct, 2007
Speaker: Wolfgang Nonner
Abstract
Ion channels are transmembrane proteins that provide a pore for ions to diffuse across the phospholipid insulator. Ion channels involved in electric signaling typically are selective for one of the hysiological ions Na+, K+, or Ca2+. This selectivity is due to a segment of the pore about 1 nm in length. It appears that structural themes, like pore or selectivity filter, have been used by nature also in the construction of transmembrane proteins with more complex transport functions than channels. Secondary transporter proteins use a pre-existing gradient of Na+ or H+ across the membrane to promote the flow of another ion species such as Ca2+. An example is the 'Na+/Ca2+ antiport' which uses the flow of Na+ to drive a counterflow of Ca2+. Physiologically, an inflow of Na+ into the cell extrudes Ca2+ from the cell. This transport system has been extensively studied because the transport activity can be monitored as an electric current. The transporter maintains an almost stoichiometric ratio between the numbers of Na+ and Ca2+ that it moves: 3 Na+ versus 1 Ca2+. Thus one net charge crosses the membrane and can be measured as current. Experiments involving variations of Na+ and Ca2+ concentrations in the baths facing the transporter and of the transmembrane voltage will be discussed. A detailed chemical-kinetic model of the transporter has been developed, which involves a pore-like transport path for the ions that is reciprocally accessible to one bath or the other, but not to both baths simultaneously. Physical mechanisms that might underly the selectivity and varied access in this pore, and approaches to modeling such mechanisms, will be discussed.
URL: www.ricam.oeaw.ac.at/specsem/ssqbm/participants/abstracts/index.php
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