Membrane Proteins Can Be Associated with the Lipid Bilayer in Various Ways
Different membrane proteins are associated with the membranes in different ways, as illustrated in Figure 10-17. Many extend through the lipid bilayer, with part of their mass on either side (examples 1, 2, and 3 in Figure 10-17). Like their lipid neighbors, these transmembrane proteins are amphipathic, having regions that are hydrophobic and regions that are hydrophilic. Their hydrophobic regions pass through the membrane and interact with the hydrophobic tails of the lipid molecules in the interior of the bilayer, where they are sequestered away from water. Their hydrophilic regions are exposed to water on either side of the membrane. The hydrophobicity of some of these transmembrane proteins is increased by the covalent attachment of a fatty acid chain that inserts into the cytosolic monolayer of the lipid bilayer (example 1 in Figure 10-17).
Other membrane proteins are located entirely in the cytosol and are associated with the cytosolic monolayer of the lipid bilayer either by an amphipathic α helix exposed on the surface of the protein (example 4 in Figure 10-17) or by one or more covalently attached lipid chains, which can be fatty acid chains or prenyl groups (example 5 in Figure 10-17 and Figure 10-18). Yet other membrane proteins are entirely exposed at the external cell surface, being attached to the lipid bilayer only by a covalent linkage (via a specific oligosaccharide) to phosphatidylinositol in the outer lipid monolayer of the plasma membrane (example 6 in Figure 10-17).
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The lipid-linked proteins in example 5 in Figure 10-17 are made as soluble proteins in the cytosol and are subsequently directed to the membrane by the covalent attachment of a lipid group (see Figure 10-18). The proteins in example 6, however, are made as single-pass transmembrane proteins in the ER. While still in the ER, the transmembrane segment of the protein is cleaved off and a glycosylphosphatidylinositol (GPI) anchor is added, leaving the protein bound to the noncytosolic surface of the membrane solely by this anchor (discussed in Chapter 12). Proteins bound to the plasma membrane by a GPI anchor can be readily distinguished by the use of an enzyme called phosphatidylinositol-specific phospholipase C. This enzyme cuts these proteins free from their anchors, thereby releasing them from the membrane.
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Some membrane proteins do not extend into the hydrophobic interior of the lipid bilayer at all; they are instead bound to either face of the membrane by noncovalent interactions with other membrane proteins (examples 7 and 8 in Figure 10-17). Many of the proteins of this type can be released from the membrane by relatively gentle extraction procedures, such as exposure to solutions of very high or low ionic strength or of extreme pH, which interfere with protein-protein interactions but leave the lipid bilayer intact; these proteins are referred to as peripheral membrane proteins. Transmembrane proteins, many proteins held in the bilayer by lipid groups, and some proteins held on the membrane by unusually tight binding to other proteins cannot be released in these ways. These proteins are called integral membrane proteins.
How a membrane protein associates with the lipid bilayer reflects the function of the protein. Only transmembrane proteins can function on both sides of the bilayer or transport molecules across it. Cell-surface receptors are transmembrane proteins that bind signal molecules in the extracellular space and generate different intracellular signals on the opposite side of the plasma membrane. Proteins that function on only one side of the lipid bilayer, by contrast, are often associated exclusively with either the lipid monolayer or a protein domain on that side. Some of the proteins involved in intracellular signaling, for example, are bound to the cytosolic half of the plasma membrane by one or more covalently attached lipid groups.
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