Structure and Mechanism of Membrane Transporters

Research

Membrane transport in cells is a fundamental biological process that is mediated by various channel and transporter proteins. A major type of such proteins is secondary active membrane transporters, which use a solute gradient to drive the translocation of other substrates. The largest secondary transporter protein family known is the major facilitator superfamily (MFS), with more than one hundred thousand members identified to date. These proteins transport ions, sugars, sugar-phosphates, drugs, neurotransmitters, nucleosides, amino acids, peptides, and other hydrophilic solutes. Members of this superfamily are ubiquitous in all three kingdoms of living organisms, and many have medical or pharmacological relevance. For example, the mammalian glucose transporter Glut4 from muscle and adipose cells is responsible for their glucose uptake, a process that is impaired in type II diabetes. Inherited mutations in a related transporter, Glut1 from erythrocyte and brain blood barrier, cause Glucose Transporter 1 Deficiency Syndrome, a disease whose symptoms include infantile seizures and developmental delay. Similarly, mutations in human glucose-6-phospahte transporter (G6PT) cause glycogen storage disease type 1b. In bacteria, MFS proteins function principally for nutrient uptake, but some act as drug efflux pumps that confer antibiotic resistance.

We attempt to understand the mechanism of the MFS proteins by determining the crystal structure of a member of the transporter family. We crystallized GlpT, the glycerol-3-phosphate (G3P) transporter from theE. coli inner membrane. In E. coli, G3P serves both as a carbon and energy source and as a precursor for phospholipid biosynthesis. GlpT is an organic phosphate/inorganic phosphate (Pi) antiporter that functions for G3P uptake and is driven by a Pi gradient. We have determined the structure of GlpT at 3.3 Å resolution by X-ray crystallography. This is the first MSF structure ever determined to high resolution. The transporter structure suggests that the substrate translocation is by a single-binding site, alternating-access mechanism via a rocker-switch type of movement of the N- and C-terminal domains of the protein.

The lab is equipped with up-to-date equipment for carrying out structural biology research, from cell culture, molecular biology and biochemistry, to X-ray Crystallography and Cryo-Electron Microscopy.