How and why are channels in transporters?

Synergistic Malaria Parasite Killing by Two Types of Plasmodial Surface Anion Channel Inhibitors

Malaria parasites increase their host erythrocyte's permeability to a broad range of ions and organic solutes. The plasmodial surface anion channel (PSAC) mediates this uptake and is an established drug target. Development of therapies targeting this channel is limited by several problems including interactions between known inhibitors and permeating solutes that lead to incomplete channel block. Here, we designed and executed a high-throughput screen to identify a novel class of PSAC inhibitors that overcome this solute-inhibitor interaction. These new inhibitors differ from existing blockers and have distinct effects on channel-mediated transport, supporting a model of two separate routes for solute permeation though PSAC. Combinations of inhibitors specific for the two routes had strong synergistic action against in vitro parasite propagation, whereas combinations acting on a single route produced only additive effects. The magnitude of synergism depended on external nutrient concentrations, consistent with an essential role of the channel in parasite nutrient acquisition. The identified inhibitors will enable a better understanding of the channel's structure-function and may be starting points for novel combination therapies that produce synergistic parasite killing.

Mechanisms of glutamate transport

L-Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and plays important roles in a wide variety of brain functions, but it is also a key player in the pathogenesis of many neurological disorders. The control of glutamate concentrations is critical to the normal functioning of the central nervous system, and in this review we discuss how glutamate transporters regulate glutamate concentrations to maintain dynamic signaling mechanisms between neurons. In 2004, the crystal structure of a prokaryotic homolog of the mammalian glutamate transporter family of proteins was crystallized and its structure determined. This has paved the way for a better understanding of the structural basis for glutamate transporter function. In this review we provide a broad perspective of this field of research, but focus primarily on the more recent studies with a particular emphasis on how our understanding of the structure of glutamate transporters has generated new insights.

Glutamate and monoamine transporters: new visions of form and function

Neurotransmitters are rapidly removed from the extracellular space primarily through the actions of plasma membrane transporters. This uptake process is not only essential in the termination of neurotransmission but also serves to replenish intracellular levels of transmitter for further release. Neurotransmitter transporters couple the inward movement of substrate to the movement of Na(+) down a concentration gradient and, in addition to their transport function, some carriers also display channel-like activities. Five Na(+)/K(+)-dependent glutamate transporter subtypes belong to the solute carrier 1 (SLC1) family and a second family, SLC6, encompasses the Na(+)/Cl(-)-dependent transporters for dopamine, 5-hydroxytryptamine (serotonin), noradrenaline, GABA and glycine. Recent advances, including high-resolution structures from both families, are now providing new insights into the molecular determinants that contribute to substrate translocation and ion channel activities. Other influential studies have explored how cellular regulatory mechanisms modulate transporter function, and how the different functions of the carrier shape the patterns of neurotransmitter signaling. This review focuses on recent studies of glutamate and monoamine transporters as prototypes of the two carrier families.

Solute-inhibitor interactions in the plasmodial surface anion channel reveal complexities in the transport process

Human red blood cells infected with the malaria parasite Plasmodium falciparum have markedly increased permeabilities to diverse organic and inorganic solutes. The plasmodial surface anion channel (PSAC), recently identified with electrophysiological methods, contributes to the uptake of many small solutes. In this study, we explored the effects of known PSAC antagonists on transport of different solutes. We were surprised to find that the transport of two solutes, phenyltrimethylammonium and isoleucine, was only partially inhibited by concentrations of three inhibitors that abolish sorbitol or alanine uptake. Residual uptake via endogenous transporters could not account for this finding because uninfected red blood cells (RBCs) do not have adequate permeability for these solutes. In infected RBCs, the residual uptake of these solutes could be abolished by higher concentrations of specific and nonspecific PSAC antagonists. Adding sorbitol or alanine, permeant solutes that do not exhibit residual uptake, could also abolish it. The residual uptake did not exhibit anomalous mole fraction behavior and had a steep activation energy. These observations exclude uptake via unrelated pathways and instead point to differences in how PSAC recognizes and transports various solutes. We propose a possible model that also may help explain the unique selectivity properties of PSAC.