Bacteriorhodopsin trimeric structure

Figure 2.5 ILLustration of a structure determined from analysis of 2D crystals at 3 A resolution. Surface shaded views of (a) the cytoplasmic side and (b) the extracellular side of the bacteriorhodopsin trimer. Blue and red areas indicate positive and negative surface charges respectively. The arrow indicates the opening of the only proton channel that can be seen in this view. [Reproduced from Fujiyoshi, Y. (1999) Faseb J. 13 (suppl 2) S191-194]...

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. 

On the basis of novel electron diffraction methods, Unwin and Henderson have shown that each protein molecule consists of seven ot-hexical rods, about 40 A long and 10 A apart, all perpendicular to the plane of the membrane (39,40). The lattice structure results in clustering of the BR molecules as trimers around one of the threefold axes. Analysis of CD spectra in terms of exciton interactions between the chromophores (41,42), as well as linear dichroism measurements (43-45), have yielded values of 20-24° for the tilt angle (out of the membrane plane) of the retinal transition moment, which closely coincides with the long axis of the molecule. The absence of any rotational mobility of bacteriorhodopsin in the purple membrane (on a time scale of 60 min) was also confirmed by linear dichroism measurements (45). 

The arrangement of bacteriorhodopsin into trimers and the two-dimensional extended hexagonal lattice formed by the trimers has made it possible to describe the structure... 

In a related work, NMR spectra of [3- C]Ala- and [l- C]Val-labelled bacteriorhodopsin and a variety of its mutants have also been reported. These studies were undertaken in order to clarify contributions of the extracellular Glu residues to the conformation and dynamics of bacteriorhodopsin. Significant dynamic changes were induced for the triple or quadruple mutants, as shown by broadened NMR peaks of [l- C]Val-labelled proteins. These changes were due to acquired global fluctuation motions of the order of 10 -10" s as a result of disorganised trimeric form. It was concluded that the Glu residues at the extracellular surface play an important role in maintaining the native secondary structure of bacteriorhodopsin in the purple membrane. 

The purple membrane was one of the very first membranes to be characterised by AFM [169] here trimers of bacteriorhodopsin (BR) were observed on membranes of Halobacterium halobium. BR serves as a light-driven proton (H ) pump. It contains eight typtophan residues and is composed of seven heUces [170]. Both the membrane and BR have been the subject of a great deal of research [83,171-177] into the detailed structure of the membrane, which consists of approximately 75% BR and 25% lipids [178]. Worcester et al. [179] presented a study of the purple membrane, using platinum/palladium (80/20) wire cantilevers to study the membrane in air at a controlled relative humidity (55-75%). This study enabled the observation of unidirectional parallel rows, spaced approximately 5 nm apart from each other. It was suggested that they directly represented the smface structure of the purple membrane. 

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