Abstract
Mutations in the PfCRT protein cause chloroquine resistance (CQR), and earlier studies from our laboratory using plasma membrane inside-out vesicles (ISOV) prepared from yeast expressing recombinant PfCRT [Zhang, H., et al. (2004) Biochemistry 43, 8290-8296] suggested that the putative transporter mediates downhill facilitated diffusion of charged chloroquine (CQ). However, more recent experiments with a fluorescent CQ probe (NBD-CQ) presented in the accompanying paper (DOI 10.1021/bi901034r ) indicated that the CQR phenotype in live parasites is associated with a reduced rate of ATP-dependent CQ uptake into the digestive vacuole (DV). An altered rate constant for uptake has multiple interpretations. To further investigate this phenomenon, PfCRT proteins found in chloroquine-sensitive (CQS) and CQR strains of Plasmodium falciparum were purified from yeast engineered to express "yeast optimized" pfcrt genes, reconstituted into proteoliposomes (PL), and efflux of NBD-CQ was measured from these PL. A membrane-impermeant quencher was used to distinguish intra-PL NBD-CQ from extra-PL NBD-CQ vs time as well as resolve initial rates and rate constants for efflux. Efflux was investigated at a range of NBD-CQ concentrations, in the presence vs absence of pH gradients (DeltapH) and transmembrane potentials (DeltaPsi). Explicit turnover numbers for apparent PfCRT-mediated transport were then calculated under these conditions. Our data are consistent with a model wherein PfCRT catalyzes electrochemically downhill diffusion of NBD-CQ out of the DV, in response to DeltaPsi or DeltapH, at a rate that can partially compete with the ATP-dependent uptake of NBD-CQ by CQS parasites described in the previous paper. These data allow us to propose a refined model for altered CQ accumulation in CQR malarial parasites.