Purple membrane protein, domain structure

Indeed, hydrophilic N- or C-terminal ends and loop domains of these membrane proteins exposed to aqueous phases are able to undergo rapid or intermediate motional fluctuations, respectively, as shown in the 3D pictures of transmembrane (TM) moieties of bacteriorhodopsin (bR) as a typical membrane protein in the purple membrane (PM) of Halobacterium salinarum.176 178 Structural information about protein surfaces, including the interhelical loops and N- and C-terminal ends, is completely missing from X-ray data. It is also conceivable that such pictures should be further modified, when membrane proteins in biologically active states are not always present as oligomers such as dimer or trimer as in 2D or 3D crystals but as monomers in lipid bilayers. 

Fig. 6, left shows an end view of a type-I crystal formed by stacking two-dimensional crystal layers, ordered sheets of proteins. Many proteins, but not all, can form such a two-dimensional crystal layer, in which the hydrophobic regions of the proteins interact with the hydrocarbon tails of the lipids, the two-dimensional structure being stabilized by both hydrophobic and polar interactions. In each two-dimensional crystal layer no detergent is present and only the polar domains are exposed at the surface. These two-dimensional crystal layers then stack up to form a three-dimensional crystal through polar attractions between the layers. In three-dimensional crystals, the successive two-dimensional crystal layers need to be ordered in the third dimension with respect to translation, rotation and up-down orientation. Examples of type-I crystals which have been prepared are mitochondrial cytochrome oxidase, chloro-plastChl-a/ proteins, and a protein from the purple membrane ofhalobacteria. Two-dimensional crystals are usually rather small and useful only for examination by electron microscopy. 

The quadrupolar contribution is mostly expected in membranes with a high protein concentration, where ordered arrays of integral proteins exist. Examples of this type include the purple membranes of Halobacterium halo-hium, the inner mitochondrial membrane, etc. The presence in biomembranes of extended domains of tightly packed globular proteins in a doubletiered pattern is a basic idea in the structure-function unitization model of biomembranes. The estimated flexocoefScient of an array of identical double-tiered quadrupolar proteins is substantial > 4.5 X 10- ° C as... 

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