Membrane proteins, crystallization

Figure 12.2 (a) Schematic drawing of membrane proteins in a typical membrane and their solubilization by detergents. The hydrophilic surfaces of the membrane proteins are indicated by red. (b) A membrane protein crystallized with detergents bound to its hydrophobic protein surface. The hydrophilic surfaces of the proteins and the symbols for detergents are as in (a). (Adapted from H. Michel, Trends Biochem. Sci. 8 56-59, 1983.)... 

Membrane integral proteins have transmembrane domains that insert directly into lipid bilayers. Transmembrane domains (TMDs) consist predominantly of nonpolar amino acid residues and may traverse the bilayer once or several times. High-resolution structural information is available for only a few integral membrane proteins, primarily because it is difficult to obtain membrane protein crystals that are adequate for X-ray diffraction measurements. 

Wiener MC. 2004. A pedestrian guide to membrane protein crystallization. Methods 34 364-372. 

FIGURE 3.16 Example of high-resolution AFM imaging of a biological surface. Contact mode AFM image of Aquaporin-Z membrane protein crystals showing their surface structure with <1 nm resolution. Scale bar, 10 nm. 

X-Ray diffraction analysis is a suitable and convenient method for obtaining exact structures of membrane proteins, but it requires three-dimensional crystals. Membrane protein crystallization has always... 

Pautsch, A., Vogt,J., Model, K., Siebold, C., and Schulz, G. E. (1999). Strategy for membrane protein crystallization exemplified with OmpA and OmpX. Proteins Struct. Fund. Genet. 34, 167-172. 

Lancaster, C. R. D. (2002d). In Methods and Results in Membrane Protein Crystallization (S. Iwata, Ed.). International Univ. Line, La Jolla, CA. (in press). 

Ai, X., Caeerey, M., Membrane protein crystallization in lipidic mesophases Detergent effects. Biophys. J. 2000, 79(1), 394-405. 

In the currently accepted theories on membrane protein crystallization, discussed above, the only role of the detergent is to stabilize the membrane protein in aqueous solution. However, the effect of detergent structure on the crystallization of bovine heart cytochrome c oxidase. 

Membrane protein crystals have significantly more solvent (64%) content than soluble proteins [47% (44)] presumably because of the detergent in the crystal. The orgaiuzation of the detergent in the membrane protein crystal has been investigated in a select few cases and is different in each case. In the LH2 crystal, the detergent forms a belt around the hydrophobic surface of the protein consistent with the dimension of the OG... 

Lipid cubic (51) and sponge (52) phases, as well as bicelles (53), are alternatives to detergents that have been applied successfully to membrane protein crystallization. In these instances, the protein is embedded in a lipid bilayer environment, which is considered more natural compared with the detergents that form micellar phases. In the recent high-resolution crystal structure of the human 32 adrenergic G-protein-coupled receptor, lipid cubic phase was used with necessary cholesterol and 1,4-butandiol additives (54). The cholesterol and lipid molecules were important in facilitating protein-protein contacts in the crystal. 

Savage DF, Anderson CL, Robles-Cohnenares Y, Newby ZE, Stroud RM. Cell-free complements in vivo expression of the E. coli membrane proteome. Protein Sci. 2007 16 966-976. Wiener MC. A pedestrian guide to membrane protein crystallization. Methods 2004 34 364-372. 

Wiener MC. The development of membrane protein crystallization screens based upon detergent solution properties. Biophys. J. 2002 82 29a. 

Caffrey M. Membrane protein crystallization. J. Struct. Biol. 2003 142 108-132. 

Wiener, M. C. 2004. A pedestrian guide to membrane protein crystallization. Methods A Companion to Methods in Enzymology, Macromolecular Crystallization (ed. A. McPherson) 34(3), 364—372. 

Fig, 6. Graphical representation of two types of membrane-protein crystals. Type 1 crystal consists of stacks of two-dimensional crystalline membranes ordered in the third dimension to form the "membrane crystal. Type 11 crystals are formed by membrane proteins with the hydrocarbon tails of detergent molecules bound to their hydrophobic surfaces. The hydrophilic surfaces of the protein are indicated by dotted boundaries. Figure source Michel (1983) Crystallization of membrane proteins. Trends Biochem... 

H Michel (1990) General and practical aspects of membrane protein crystallization. In H Michel (ed) Crystallization of Membrane Proteins, pp. 74-88. CRC Press... 

Hitscherich, C., Kaplan, J., Allaman, M., Wiencek, J., and Loll, P. J. (2000). Static light scattering studies of OmpF porin imphcations for integral membrane protein crystallization, Protein, 9, 1559-1566. 

Essentially the same consideration appUes to an inverted bicontinuous cubic phase (Qn) and an inverted hexagonal (Hn) phase, which are usually formed with lipids with long hydrophobic chains. Qn and Hn phases have recently received growing attention in the pharmaceutical or biological fields, for instance, as new carriers for drug-deUvery systems, and matrices for membrane protein crystallization [37-42]. The conventional (e.g., monoolein) Qn phase, however, often transforms into a soUd phase at low temperatures around 4°C [43-47], where temperature-sensitive proteins or actives are most preferably handled and preserved. It has recently been confirmed that isoprenoid-chained lipids can in fact give a range of Qn phases that are stable at low temperatures [13]. 

Zulauf, M., Detergent phenomena in membrane protein crystallization, in Crystallization of Membrane Proteins, Michel, H. (Ed.), CRC Press, Boca Raton, FL, 1988. 

Type II, multiple layers Photoreceptor molecules form a multilayered membrane structure or a three-dimensional membrane protein crystal. The eyespot is outwardly concave and is located close to this structure. 

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