Volume Flux Across Red Cell AQP1 and E. Coli AQPZ Water Channel Proteins Reconstituted into Planar Lipid Bilayers

Abstract

Several members of the aquaporin familiy of protein water channels have been reported to carry ionic currents. The major intrinsic protein (MIP, AQPO) of the lens functioned as an ion channel when reconstituted into planar lipid bilayers. In 0.1 M KC1, it had two preferred conductance states with amplitudes of 380 and 160 pS (Ehring et al., 1990). In contrast, Xenopus oocytes expressing AQPO failed to exhibit ion channel activity but facilitated water transport (Mulders et al., 1995). Nodulin 26 (an integral symbiosome membrane protein of nitrogen- fixing soybean nodules) was the second member of the MIP channel protein family that was reported to form ion conducting channels with a large single channel conductance and weak anion selectivity (Weaver et al., 1994). In planar bilayers reconstituted nodulin 26 channels showed sensitivity to high applied voltages, including more active gating and the tendency to occupy discrete lower subconductance states (Lee et al., 1995). However, experiments involving expression of nodulin 26 in Xenopus oocytes have led to a reevaluation of the channel properties and it is now clear that nodulin 26 possesses aquaporin activity in this system (Rivers et al., 1997).