Potential dependence of the “electrically silent” anion exchange across the plasma membrane ofXenopus oocytes mediated by the band-3 protein of mouse red blood cells

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Abstract
Mouse erythroid band-3 protein was incorporated into the plasma membrane ofXenopus oocytes by microinjection of poly(A)+-mRNA from spleens of anemic mice. Subsequently, the efflux of microinjected36Cl was continuously followed in single oocytes in a perfusion chamber the bottom of which was formed by the window of a Geiger-Müller tube. During the flux measurements, the membrane potential was clamped to different holding potentials. The efflux increased over the voltage range of −10 to −100 mV by a factor of about 1.5. Since the membrane potential cannot act as a driving force of anion exchange, it is suggested that the observed slight potential dependence is related to a recruitment of the anion-loaded transport protein by the electrical field, thereby changing the steady-state distribution between inwardly and outwardly facing anion binding sites of the transport molecules. The experimental data are discussed in terms of ping-pong kinetics, assuming that the potential dependence is primarily due to an effect of the electrical field in the membrane on the ratelimiting interconversion of inwardly and outwardly oriented anion binding sites. The results are compatible with the assumption that in the oocyte membrane the substrate-loaded band-3 molecules are preferentially inwardly oriented, and that the transition from the inwardly to the outwardly oriented conformation is associated with a reorientation of an effective charge of 0.1 elementary charge. During progesterone-induced maturation of the oocytes, several endogenous transport systems change their activity drastically. The mouse band-3 protein in the oocyte membrane also undergoes activity changes; however, these changes do not seem to involve direct regulation by specific metabolic processes. They can be explained as a consequence of the depolarization of the membrane potential associated with the maturation process.

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