A number of mitochondrial carrier systems function both in homologous and in heterologous exchange mode, which, in the case of the phosphate carrier is a homologous Pi(-)-Pi- and a heterologous Pi(-)-OH- exchange. In addition, we showed that mitochondrial carriers, e.g. the aspartate/glutamate and the phosphate carrier, can undergo a functional shift from coupled antiport to uncoupled uniport after modification of cysteine residues. In this transport mode a mixture of carrier- and channel-type properties is observed. To address this question on the molecular level, the phosphate carrier from yeast (S. cerevisiae) mitochondria was expressed in E. coli, solubilized, purified and functionally reconstituted. From three cysteine residues present in the yeast phosphate carrier at positions 28, 134 and 300, only one single cysteine residue (C28) was found responsible for the functional switch from antiport to uniport. Upon replacement by a serine residue, this interconversion was blocked. After incorporation of the carrier into giant liposomes, electrophysiological methods (patch clamp) were applied. Under these conditions, a fourth transport mode of the phosphate carrier was observed, namely an action as anion-selective channel, which could be reversibly blocked by phosphate.

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