The Composition and Architecture of Membranes:- Integral Proteins Are Held in the Membrane by Hydrophobic Interactions with Lipids

The firm attachment of integral proteins to membranes is the result of hydrophobic interactions between mem brane lipids and hydrophobic domains of the protein. Some proteins have a single hydrophobic sequence in the middle (as in glycophorin) or at the amino or carboxyl terminus. Others have multiple hydrophobic sequences, each of which, when in the -helical conformation, is long enough to span the lipid bilayer (Fig. 11–8). The same techniques used to determine the three-dimensional structures of soluble proteins can, in principle, be applied to membrane proteins. In practice, however, membrane proteins have until recently proved difficult to crystallize. New techniques are overcoming this obstacle, and crystallographic structures of mem brane proteins are regularly becoming available, yielding deep insights into membrane events at the molecular level.

One of the best-studied membrane-spanning proteins, bacteriorhodopsin, has seven very hydrophobic in ternal sequences and crosses the lipid bilayer seven times. Bacteriorhodopsin is a light-driven proton pump densely packed in regular arrays in the purple mem brane of the bacterium Halobacterium salinarum. X-ray crystallography reveals a structure with seven helical segments, each traversing the lipid bilayer, connected by nonhelical loops at the inner and outer face of the membrane (Fig. 11–9). In the amino acid sequence of bacteriorhodopsin, seven segments of about 20 hydrophobic residues can be identified, each seg ment just long enough to form an helix that spans the bilayer. Hydrophobic interactions between the nonpolar amino acids and the fatty acyl groups of the membrane lipids firmly anchor the protein in the membrane. The seven helices are clustered together and oriented not quite perpendicular to the bilayer plane, providing a transmembrane pathway for proton movement. As we shall see in Chapter 12, this pattern of seven hydrophobic membrane-spanning helices is a common motif in membrane proteins involved in signal reception. The photosynthetic reaction center of a purple bacterium was the first membrane protein structure solved by crystallography. Although a more complex membrane protein than bacteriorhodopsin, it is constructed on the same principles. The reaction center has four protein ubunits, three of which contain -helical segments that span the membrane (Fig. 11–10). These segments are rich in nonpolar amino acids, their hydrophobic side chains oriented toward the outside of the molecule where they interact with membrane lipids. The architecture of the reaction center protein is therefore the inverse of that seen in most water-soluble proteins, in which hydrophobic residues are buried within the protein core and hydrophilic residues are on the surface (recall the structures of myoglobin and hemoglobin, for example). In Chapter 19 we will encounter several complex membrane proteins having multiple transmembrane helical segments in which hydrophobic chains are positioned to interact with the lipid bilayer.

FIGURE 11–8 Integral membrane proteins. For known proteins of the plasma membrane, the spatial relationships of protein domains to the lipid bilayer fall into six categories. Types I and II have only one trans membrane helix; the amino-terminal domain is outside the cell in type I proteins and inside in type II. Type III proteins have multiple trans membrane helices in a single polypeptide. In type IV proteins, trans membrane domains of several different polypeptides assemble to form a channel through the membrane. Type V proteins are held to the bilayer primarily by covalently linked lipids (see Fig. 11–14), and type VI proteins have both transmembrane helices and lipid (GPI) anchors. In this figure, and in figures throughout the book, we represent transmembrane protein segments in their most likely conformations: as α helices of six to seven turns. Sometimes these helices are shown simply as cylinders. As relatively few membrane protein structures have been deduced by x-ray crystallography, our representation of the extramembrane domains is arbitrary and not necessarily to scale.

FIGURE 11–9 Bacteriorhodopsin, a membrane-spanning protein. (PDB ID 2AT9) The single polypeptide chain folds into seven hydrophobic helices, each of which traverses the lipid bilayer roughly perpendicular to the plane of the membrane. The seven transmem brane helices are clustered, and the space around and between them is filled with the acyl chains of membrane lipids. The light-absorbing pigment retinal is buried deep in the membrane in contact with several of the helical segments (not shown). The helices are colored to correspond with the hydropathy plot in Figure 11–11b.

FIGURE 11–10 Three-dimensional structure of the photosynthetic reaction center of Rhodopseudomonas viridis, a purple bacterium. This was the first integral membrane protein to have its atomic structure determined by x-ray diffraction methods (PDB ID 1PRC). Eleven-helical segments from three of the four subunits span the lipid bi layer, forming a cylinder 45 Å (4.5 nm) long; hydrophobic residues on the exterior of the cylinder interact with lipids of the bilayer. In this ribbon representation, residues that are part of the transmembrane helices are shown in yellow. The prosthetic groups (light-absorbing pigments and electron carriers; see Fig. 19–45) are red.

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مواضيع ذات صلة

Membrane Dynamics:- Transbilayer Movement of Lipids Requires Catalysis

The Composition and Architecture of Membranes:- Lipids and Proteins Diffuse Laterally in the Bilayer

Membrane Dynamics:- Acyl Groups in the Bilayer Interior Are Ordered to Varying Degrees

The Composition and Architecture of Membranes:- Covalently Attached Lipids Anchor Some Membrane Proteins

The Composition and Architecture of Membranes:- The Topology of an Integral Membrane Protein Can Be Predicted from Its Sequence

The Composition and Architecture of Membranes:- All Biological Membranes Share Some Fundamental Properties

The Composition and Architecture of Membranes:- Each Type of Membrane Has Characteristic Lipids and Proteins

Working with Lipids:- Mass Spectrometry Reveals Complete Lipid Structure

Working with Lipids:- Gas-Liquid Chromatography Resolves Mixtures of Volatile Lipid Derivatives

Working with Lipids:- Adsorption Chromatography Separates Lipids of Different Polarity

Working with Lipids:- Lipid Extraction Requires Organic Solvents

Lipids as Signals, Cofactors, and Pigments: -Plants Use Phosphatidylinositols, Steroids, and Eicosanoidlike Compounds in Signaling