Purple membrane of Halobacterium halobium

Bacteriorhodopsin, a light-driven proton pump, is a large (27000 Dalton) membrane protein, located in the purple membrane of halobacterium halobium (for a recent review see Lanyi, 1993). It spans the membrane by seven-a-helices (see Fig 6.6-7). The chro-mophore retinal is embedded inside the protein, shielded by the helices. Retinal connects to the Lys 216 of the protein via a protonated Schiff s base (Fig. 6.6-7). 

The purple membrane of Halobacterium halobium was discovered in the late 1960 s when Stoeckenius and co-workers described sheet-like structures which could... 

Notwithstanding this complexity, the need for three-dimensional, structural information at the atomic level of resolution is central and indispensable to biomembrane science. X-ray, and to a lesser extent neutron-diffraction, as the most important sources for such information have, therefore, been widely used in this field (for reviews, see Refs. 1-4). The success of this approach, however, has generally been less spectacular than for instance in the cases of protein or nucleic acid structure. The reasons for this lie in the very nature of biological membranes with few, notable exceptions (such as the purple membrane of halobacterium halobium, which can be viewed essentially as a two-dimensional crystal of bacteriorhodopsin with only little lipid. Refs. 5, 6,25) biological membranes are characterized by highly complex and variable molecular compositions, and by the structural dynamics, fluidity , which is in many cases essential for enzymatic, or other, functions of membranes. As a reflection of this most natural membranes do not crystallize, and a full, three-dimensional atomic structure analysis seems out of reach. 

Bacteriorhodopsin (BR) is the only membrane-bound protein in the purple membrane of Halobacterium halobium [12-16]. It is a single-chain polypeptide... 

Fig. 5. Schematic representation of the architecture of biological membranes, a. Singer and Nicolson concept, proteins are free to move. b. Integral membrane proteins are connected with the membrane skeleton by means of an ankyrin protein, as in the erythrocyte, c. Hormone- or antibody-induced aggregation of receptors resulting in a coated pit. d. Aggregation of proteins in contact sites between cells like the gap junction, e. Paracrystalline protein structures in the plane of the bilayer, as in the purple membrane of Halobacterium halobium. f. Phase separation of lipids resulting in the aggregation of proteins.

Another interesting example for severe hydration effects in relation to charge transfer is the purple membrane of Halobacterium halobium it acts as a photoelectret, producing at least semipermanent electric charges upon illumination this activity has been shown to be greatly affected by hydration. 

Neutron diffraction peaks from oriented sheets of the purple membrane of Halobacterium halobium were measured with different levels of HjO/DjO exchange. ... 

There are two ways in which membranes of diacetylenic lipids containing intrinsic membrane proteins can be obtained either proteins extracted from natural membranes with detergent can be reconstituted into synthetic diacetylenic phosphatidylcholines or the growth medium of micro-organisms incapable of synthesizing their own fatty acids can be enriched with diacetylenic fatty acid. In this laboratory, Ca2+-ATPase from sarcoplasmic reticulum and bacteriorhodopsin from the purple membrane of Halobacterium halobium have been reconstituted into diacetylenic phosphatidylcholines. Provided the more reactive mixed-chain lipids are used polymerisation can be achieved before the protein is denatured by the UV irradiation. Both proteins remain active within polymeric bilayers. 

The photocycle of bacteriorhodopsin, the protein pigment of the purple membrane of Halobacterium halobium has generated considerable interest in the field of molecular photobiology, (see Ref. 2-4 for recent views). This is due to the close relationship between its photocycle and the mechanism of the light-driven proton pump ) which is capable of driving ATP synthesisand also to the striking structural and photophysical analogies between bacteriorhodopsin... 

Oesterhelt, D. and W. Stoeckenius, Rhodopsin-hke protein from the purple membrane of Halobacterium halobium. Nature (London), New Biol, 1971,233 149-152. 

M. B. Jackson and J. M. Sturtevant, Phase transitions of the purple membranes of Halobacterium halobium. Biochemistry, 17, 911 (1978). 

Oesterhelt, D., Stoeckenius, W. (1971). Rhodopsin-like protein from the purple membrane of halobacterium halobium. Nature New Biology, 233,149-152. 

G. Varo, L. Keszthelyi, Photoelectric signals from dried oriented purple membranes of Halobacterium halobium, Biophys. J. 43 (1983) 47-51. 

Although the general concept of long-lived boundary lipids surrounding intrinsic proteins does not appear to be viable at present, there are well documented cases where particular lipids are associated with particular proteins. For example, the purple membranes of Halobacterium halobium apart from having a rigidly defined protein structure also contain a unique lipid composition. The lipids appear to be interdigitated between the protein rods and have a composition quite distinct from the rest of the membrane. 

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