The “purple membrane” of the archaeon Halobacterium salinarum is a specialized patch in the plasma membrane that contains a single species of protein molecule, bacteriorhodopsin (Figure 10–30A). The protein functions as a light-activated H+ pump that transfers H+ out of the archaeal cell. Because the bacteriorhodopsin molecules are tightly packed and arranged as a planar two-dimensional crystal (FIgure 10–30B and C), it was possible to determine their three-dimensional structure by combining electron microscopy and electron diffraction analysis—a procedure called electron crystallography, which we
Figure 10–29 Model of a membrane protein reconstituted into a nanodisc. When detergent is removed from a solution containing a multipass membrane protein, lipids, and a protein subunit of the high-density lipoprotein (hDl), the membrane protein becomes embedded in a small patch of lipid bilayer, which is surrounded by a belt of the hDl protein. In such nanodiscs, the hydrophobic edges of the bilayer patch are shielded by the protein belt, which renders the assembly water-soluble.
mentioned earlier. This method has provided the first structural views of many membrane proteins that were found to be difficult to crystallize from detergent solutions. For bacteriorhodopsin, the structure was later confirmed and extended to very high resolution by x-ray crystallography.
Each bacteriorhodopsin molecule is folded into seven closely packed trans-membrane α helices and contains a single light-absorbing group, or chromophore (in this case, retinal), which gives the protein its purple color. Retinal is vitamin A in its aldehyde form and is identical to the chromophore found in rhodopsin of the photoreceptor cells of the vertebrate eye (discussed in Chapter 15). Retinal is covalently linked to a lysine side chain of the bacteriorhodopsin protein. When activated by a single photon of light, the excited chromophore changes its shape and causes a series of small conformational changes in the protein, resulting in the transfer of one H+ from the inside to the outside of the cell (Figure 10–31A). In bright light, each bacteriorhodopsin molecule can pump several hundred protons per second. The light-driven proton transfer establishes an H+ gradient across the plasma membrane, which in turn drives the production of ATP by a second protein in the cell’s plasma membrane. The energy stored in the H+ gradient also drives other energy-requiring processes in the cell. Thus, bacteriorhodopsin converts solar energy into a H+ gradient, which provides energy to the archaeal cell.
The high-resolution crystal structure of bacteriorhodopsin reveals many lipid molecules bound in specific places on the protein surface (Figure 10–31B).
hydrophobic core of lipid bilayer (3 nm)
Figure 10–30 Patches of purple membrane, which contain bacteriorhodopsin in the archaeon Halobacterium salinarum. (A) these archaea live in saltwater pools, where they are exposed to sunlight. they have evolved a variety of light-activated proteins, including bacteriorhodopsin, which is a light-activated h+ pump in the plasma membrane. (B) the bacteriorhodopsin molecules in the purple membrane patches are tightly packed into two-dimensional crystalline arrays. (c) Details of the molecular surface visualized by atomic force microscopy. With this technique, individual bacteriorhodopsin molecules can be seen. (D) Outline of the approximate location of the bacteriorhodopsin monomer and the individual α helices in the image shown in (c). (B–c, courtesy of Dieter Oesterhelt; D, pDB code: 2BrD.)