Tryptophan 224 of the rat mitochondrial carnitine/acylcarnitine carrier is crucial for the antiport mechanism

The mitochondrial carnitine/acylcarnitine carrier (CACT) catalyzes an antiport of carnitine and acylcarnitines and also a uniport reaction with a rate of about one tenth with respect to the antiport rate. The antiport process results from the coupling of the two uniport reactions in opposite directions. In this mechanism, the transition of the carrier from the outward open conformation to the inward open one (or vice versa) is much faster for the carrier-substrate complex than for the unbound carrier. To investigate the molecular determinants that couple the binding of the substrate with the conformational transitions, site directed mutagenesis has been employed. The antiport or the uniport reaction was followed as [3H]carnitine uptake in or efflux from proteoliposomes reconstituted with the WT or Trp mutants of the rat CACT. Substitution of each the three Trp residues led to different results. Nearly no variations were observed upon substitution of W192 and/or W296 with Ala. While, substantial alteration of the transport function was observed in the mutants W224A, W224Y and W224F. Mutation of W224 led to the loss of the antiport function while the uniport function was unaltered. In these mutants impairment of the substrate affinity on the external side was also observed. The data highlights that W224 is involved in the coupling of the substrate binding with the matrix gate opening. The experimental data are in line with predictions by homology modeling of the CACT in its cytosolic (c-state) or matrix (m-state) opened conformations.