Membrane Pores, Channels, Pumps and Transporters

Biological membranes form a semi-permeable barrier surrounding all living cells. The types of molecules that can enter or exit the cell are defined by the integral proteins embedded in the membrane, which have very specific substrate specificities and are often regulated. Membrane proteins either form a pore or a channel, which allow the passive diffusion of a molecule along its concentration gradient, or they constitute energy-dependent pumps or transporters, which transport molecules against their concentration gradient. To fully understand the function of a membrane protein, knowledge of its structure is absolutely essential. Only then can we hope to resolve questions concerning substrate specificity, regulation and energy coupling. Although an estimated third of all proteins are membrane proteins, only a very limited number of membrane protein structures are available. This is due to the amphipathic nature of membrane proteins, which poses unique problems to determining their structure by X-ray crystallography and NMR spectroscopy. Electron microscopy is a viable alternative to these more established techniques to visualize the structure of membrane proteins. Electron crystallography of two-dimensional (2D) crystals can achieve near-atomic resolution and has already been used to determine the structure of a number of membrane proteins. We have substantial experience in growing 2D crystals and we attempt to use electron crystallography to determine the structure of a membrane protein whenever we have sufficient quantities to perform 2D crystallization trials. We also use single particle electron microscopy to study gross conformational changes in membrane proteins related to their function and to visualize complexes formed by a membrane protein with other proteins.

Aquaporins