Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

586 Chapter 10: Membrane Structure
high-density
lipoprotein
“belt”
nanodisc
phospholipids
membrane protein
in nanodisc
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.
5 nm
ATP (ATP synthases) use H + gradients in mitochondrial, chloroplast, and bacterial
membranes to produce ATP.
Membrane proteins can also be reconstituted from detergent solution into
nanodiscs, which are small, uniformly sized patches of membrane that are surrounded
by a belt of protein, which covers the exposed edge of the bilayer to keep
the patch in solution (Figure 10–29). The belt is derived from high-density lipoproteins
(HDL), which keep lipids soluble for transport in the blood. In nanodiscs
MBoC6 n10.100/10.31.5
the membrane protein of interest can be studied in its native lipid environment
and is experimentally accessible from both sides of the bilayer, which is useful,
for example, for ligand-binding experiments. Proteins contained in nanodiscs can
also be analyzed by single particle electron microscopy techniques to determine
their structure. By this rapidly improving technique (discussed in Chapter 9), the
structure of a membrane protein can be determined to high resolution without a
requirement of the protein of interest to crystallize into a regular lattice, which is
often hard to achieve for membrane proteins.
Detergents have also played a crucial part in the purification and crystallization
of membrane proteins. The development of new detergents and new expression
systems that produce large quantities of membrane proteins from cDNA
clones has led to a rapid increase in the number of three-dimensional structures
of membrane proteins and protein complexes that are known, although they are
still few compared to the known structures of water-soluble proteins and protein
complexes.
Bacteriorhodopsin Is a Light-driven Proton (H + ) Pump That
Traverses the Lipid Bilayer as Seven α Helices
In Chapter 11, we consider how multipass transmembrane proteins mediate
the selective transport of small hydrophilic molecules across cell membranes.
But a detailed understanding of how such a membrane transport protein works
requires precise information about its three-dimensional structure in the bilayer.
Bacteriorhodopsin was the first membrane transport protein whose structure was
determined, and it has remained the prototype of many multipass membrane
proteins with a similar structure.
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