For example, the structures of OmpF porin from Escherichia coli in different detergents and crystal forms revealed some interesting aspects about detergent behavior. Pebay-Peyroula et al. When Penel et al. Moreover, detergent-detergent interactions are often an integral part of the long range structure in membrane protein crystals. A view of membrane protein interactions with lipids.
Native lipids are seen bound to the surface of bacteriorhodopsin in the 1. If detergent interactions and structure play a role in membrane protein crystal growth and integrity, could more lipid-like surfactants serve the same role? Landau and Rosenbusch proposed this question and came up with a novel way of crystallizing membrane proteins 40 , In essence, a preformed surfactant phase with a more membrane-like structure might be used to partition membrane proteins into an environment that would favor close interactions suitable for nucleating and sustaining crystal growth.
The bicontinuous cubic surfactant phases made by monoacyl glycerols 16 , 42 seem ideal for this purpose as continuous regions of solvent and surfactant extend throughout the phase and can co-exist with a bulk solvent phase. Detergent-solubilized membrane protein, added externally, can easily partition into the bicontinuous cubic phase; the solvent channels allowed the manipulation of the aqueous environment to initiate crystallization.
Although many of the assumptions made by Landau and Rosenbusch are not confirmed, their technique allowed the high resolution structure determination of bacteriorhodopsin 43 , 44 and halorhodopsin The crystal structure of bacteriorhodopsin obtained from the cubic phase system discussed above 43 , 44 showed a remarkable feature: a layer of lipid molecules was resolved on the protein surface. The nature of the lipids, originating from the native bacterial membrane, and their positioning in the grooves and crevices of the protein Fig.
Over the years, numerous studies have demonstrated that membrane lipids are rapidly exchanging at the surface of integral membrane proteins 46 , even though a motionally restricted population was observed and quantified by EPR With the advent of high resolution crystal structures of membrane proteins, the observation of protein-bound lipid molecules now appears to be becoming a rule rather than an exception.
Moreover, these crystalline complexes of membrane proteins and lipid do not contain just unusual lipids, such as cardiolipin 49 or diether lipids 44 , but also more common phospholipids. The structure of bovine cytochrome c oxidase at 2. At higher resolution 2. These recent crystallographic results imply that lipid may help membrane proteins assume more stable and homogeneous conformations.
Hence, many detergents may work best along with retention of some native lipid In contrast, complete lipid removal demands that a detergent must be able to substitute successfully for most, if not all, bound lipid e. Nonetheless, the maintenance of some lipid-protein interactions may be critical for procedures like crystallization. The significance of these findings is profound in terms of how we approach the use of detergents in purification.
As mentioned earlier, the complete removal of lipid to obtain monodisperse, homogeneous PDCs was an early goal for x-ray crystallography or NMR to minimize self-association into insoluble, polydisperse aggregates 28 , which is often promoted by phospholipid. However, complete removal of bound lipid from many membrane proteins is rarely achieved and is often detrimental to structure and function 13 , 57 , For bacteriorhodopsin, NMR studies 59 clearly showed changes as native lipid was removed.
Finally, conditions and detergents that can maintain native-like activity 60 , 61 may still induce subtle changes that are not detectable in routine assays 57 , 62 , Hence, complete delipidation may not be the appropriate goal when designing purification procedures with the aim of structure determination The critical role of detergents in all aspects of membrane protein biochemistry cannot be fully addressed in the context of this short review.
As noted above, the behavior of detergents clearly impacts membrane protein purification and crystallization, as well as reconstitution 1 , which was not discussed. However, a few generalities can be made that apply to all systems. The nature of the solubilization detergent is an important factor in determining the size and properties of the resulting PDCs.
Moreover, the starting lipid content in the purified protein is a critical but often uncontrolled variable. Banerjee et al.
James U. Bowie - Publications
Such careful studies may be de rigueur for the successful structural analysis of many membrane proteins. We thank Drs. Bogusz, R. Venable, and R. We also thank Dr. Yoshikawa for permission to discuss unpublished observations. This minireview is dedicated to Drs.
Jacqueline A. Reynolds and the late Martin Zulauf who gave one of us R.
Yoshikawa, personal communication. You'll be in good company. Journal of Lipid Research.
The role of hydrophobic interactions in positioning of peripheral proteins in membranes
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Detergents and Lipids as Surfactants
Zulauf M. CRC Press, Inc. Boca Raton, FL , pp 54 — New York. Rosen M.
- Loves Beginning (A Love Rekindled Book 1).
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Reports 52 : 1 — Gunnarsson G. Haneskog L. Acta : 39 — Marone P. Crystal Growth : — Mitchell D. Faraday Trans. Bogusz S. B : — Tieleman D. Menger F. Thomas M. Zhou C. Nilsson P. Lambert O. Bordier C. Sivars U. Weckstrom K. Kragh-Hansen U.
Claudins: Volume 65
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