Bacteriorhodopsin amino-acid sequence

FIGURE 2.3 The three main families of mammalian G-protein-coupled 7TM receptors in mammals. No obvious sequence identity is found between the rhodopsin-like family A, the glucagon/VIP/calcitonin family B, and the metabotropic glutamate/chemosensor family C of G-protein-coupled 7TM receptors, with the exception of the disulfide bridge between the top of TM-III and the middle of extracellular loop-2 (see Figure 2.2). Similarly, no apparent sequence identity exists among members of these three families and, for example the 7TM bitter taste receptors, the V1R pheromone receptors, and the 7TM frizzled proteins, which all are either known or believed to be G-protein-coupled receptors. Bacteriorhodopsins, which are not G-protein-coupled proteins but proton pumps, are totally different in respect to amino-acid sequence but have a seven-helical bundle arranged rather similarly to that for the G-protein-coupled receptors. 

On the basis of their amino acid sequences and hydropathy plots, many of the transport proteins described in this chapter are believed to have multiple membrane-spanning helical regions—that is, they are type III or type IV integral proteins (Fig. 11-8). When predictions are consistent with chemical studies of protein localization (such as those described above for glycophorin and bacteriorhodopsin), the assumption that hydrophobic regions correspond to membrane-spanning domains is much better justified. 

Halorhodopsiti. In addition to bacteriorhodopsin there are three other retinal-containing proteins in membranes of halobacteria. From mutant strains lacking bacteriorhodopsin the second protein, halorhodopsin, has been isolated. It acts as a light-driven chloride ion pump, transporting Cl from outside to inside. Potassium ions follow, and the pump provides a means for these bacteria to accumulate KC1 to balance the high external osmotic pressure of the environment in which they live.578 The amino acid sequences of halorhodopsins from several species are very similar to those of bacteriorhodopsin as is the three-dimensional structure.589 However, the important proton-carrying residues D85 and D96 of bacteriorhodopsin are replaced by threonine and alanine, respectively, in halorhodopsin.590 Halorhodopsin (hR)... 

Figure 12.19 Amino acid sequence of bacteriorhodopsin. The seven helical regions are highlighted in yellow and the charged residues in red.

Initial protein structure studies indicated that bacteriorhodopsin is a 248-residue polypeptide. Its amino-terminal residue is on the membrane s outside surface, and its carboxyl residue projects into the cytoplasm. Careful analysis of bacteriorhodopsin s primary sequence revealed seven peptide segments with amino acid sequences typical of a-helices. Using electron microscopy and X-ray crystallography, researchers determined that bacteriorhodopsin possesses seven a-helices, which are roughly perpendicular to the membrane (Figure 11C). 

At this moment a large amount of information is available on the structure of bacteriorhodopsin [20] and it is very likely that this proton pump will be the first for which a detailed molecular description of its catalytic activity can be given. The amino acid sequence, the folding of the polypeptide chain and the arrangement of the protein molecules in the purple membrane are known [21,22]. The protein consists of 7 a-helices, each spanning the membrane. The retinal group is located in a hydrophobic pocket, about 15 A from the extracellular surface of the membrane. 

Fig. 1. Sequence alignment for three bacteriorhodopsins and two halorhodopsins. For brevity the single-letter amino acid code is used. Alignment and helical assignments are as in ref. [17]. Designations AR-1, a bacteriorhodopsin from Halobacterium sp. aus-1 [44] AR-2, a bacteriorhodopsin from Halobacterium sp. aus-2 [45] BR, bacteriorhodopsin from H. halobium [5] FIR, halorhodopsin from H. halobium [43] PHR, a halorhodopsin from Natronobacterium pharaonis[ l]. The patterns of dots and asterisks indicate either identity in all five sequences (dots only) or identity among the bacteriorhodopsins and halorhodopsins only (dots and asterisks).

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