Ca2+-activated synexin forms highly selective, voltage-gated Ca2+ channels in phosphatidylserine bilayer membranes.

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Abstract
Synexin, a cytosolic protein that mediates Ca2+-dependent membrane fusion, was incorporated into acidic phospholipid bilayers, formed at the tip of a patch pipet. The pipet was filled with a high-Ca2+ solution (50 mM) and immersed in a chamber containing a low-Ca2+ solution (1 mM). Brief exposures of the bilayer to synexin increased the capacitance of the bilayer by a factor of 10 and decreased the membrane resistance by a factor of 20. Reduction of Ca2+ in the chamber to 1 .mu.M caused an abrupt increase in the current required to hold the pipet potential at O mV. Under certain conditions channel events could be detected, often occurring in bursts. Consistently, open-time histograms were found to be voltage-dependent and to exhibit one time constant in the time range examined here. The slope conductance for the synexin channel was estimated at 10.2 .+-. 2.1 pS for the large Ca2+ gradient with low chamber Ca2+. However, for symmetrical, low Cl- solutions containing 25 mM Ca2+ the conductance was 26.5 .+-. 5.2 pS. Ion-replacement studies showed the synexin channel to much prefer Ca2+ over Ba2+ or Mg2+ .cntdot. Cd2+, a potent blocker of other voltage-gated Ca2+ channels at 100 .mu.M, blocked synexin channels only at very high concentrations (.gtoreq. 10 mM). Similarly, nifedipine, an inhibitor of the nonactivating Ca2+ channel, was effective only at extremely high concentrations (> 300 .mu.M). The high selectivity for Ca2+ and the lack of response of the channel to various drugs known to block Ca2+ channels thus distinguish the synexin channel from other types of Ca2+ channels hitherto reported.