Crystallisation of membrane proteins

H. Michel (Ed.), Crystallisation of Membrane Proteins, CRC Press, Boca Raton, 1991. ISBN 0849348161. 

Hunte, C., and Michel, H. (2002). Crystallisation of membrane proteins mediated by antibody fragments. Curr. Opin. Struct. Biol. 12, 503-508. 

Hunte C, Michel H. Crystallisation of membrane proteins mediated... 

X-ray crystallographic studies of such proteins obviously require that crystals of the pure protein are obtained. As a first step large quantities of a single protein need to be obtained. Some membrane proteins can be solubilised by mild treatment (e.g. with salt). Other membrane proteins are much more strongly bound and can be solubilised only by treatment with, for example, detergent which competes with the hydrocarbon chains of the membrane lipid for the non-polar interactions of the protein (figure 3.13). Crystallisation of membrane bound proteins has... 

There are several examples of parts of membrane proteins determined crystallographically. Here, a protein anchored in the membrane is clipped to provide just the hydrophilic domain for crystallisation. These studies were of cytochrome b5 (Mathews, Argos and Levine 1972), haemagglutinin (Wilson etal 1981) and neuraminidase (Varghese, Laver and Colman 1983) from influenza virus and the human class I histocompatibility antigen, HLA-A2 (Bjorkman et al 1987). 

Figure 3.14 The two basic types of membrane protein ciystals. Type I stacks of membranes containing two-dimensionally crystalline membrane proteins, which are then ordered in the third dimension. Type II a membrane protein crystallised with detergents bound to its hydrophobic surface. The polar surface part of the membrane proteins is indicated by broken lines. The symbols for liquids and detergents are the same as in figure 3.13. From Deisenhofer and Michel (1989) with the permission of the authors, EMBO J, Oxford University Press and copyright The Nobel Foundation (1989).

To date, very limited information on the atomic structure is available, since crystallisation of hydrophobic membrane proteins remains a challenging problem. 

Proteins embedded in the lipid bilayer of membranes play an important role in membrane functions, involving transport across the bilayer, electron flow and energy conversion, cell recognition, receptor functions, etc. There is not much information available on structural features of these proteins due to difficulties in crystallisation, necessary for complete structure determination. 

There have been several special cases where membrane proteins have been crystallised and high-resolution X-ray structures obtained. Where the protein is anchored in the membrane by a hydrophobic tail the protein may be released, for example, by proteases. The water-soluble component so released can be crystallised like a normal soluble globular protein. Outstanding examples of this approach have been cytochrome bi [49], influenza virus haemaglutinin [50] and influenza virus neuraminidase [51]. Alternatively, there have been a few examples of small proteins which can incorporate themselves into membranes and which are suflSciently homogeneous to be... 

Figure 2.6 The microdialysis crystallisation procedure, (a) 5fi( of a protein solution is injected into a capillary and covered by a dialysis membrane of 1x1 cm2, (b) The membrane is fastened with a piece of teflon tubing, (c) The protein solution is spun down in a table centrifuge and the capillary is closed by a piece of modelling clay, (d) Next, the capillary is placed in an Eppendorf reservoir containing the dialysis solution and capped, (e).

Figure 6.11 Dialysis Saturated solution of biological macromolecule (e.g. protein) is placed in an environment separated from precipitant solution by semi-permeable membrane. Very slow solute diffusion across the membrane creates precipitant gradient in the macromolecule solution to "seed" crystallisation.

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