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

Some membrane proteins contain one or more covalently linked lipids of several types: long-chain fatty acids, isoprenoids, sterols, or glycosylated derivatives of phosphatidylinositol, GPI (Fig. 11–14). The attached lipid provides a hydrophobic anchor that inserts into the lipid bilayer and holds the protein at the membrane sur face. The strength of the hydrophobic interaction between a bilayer and a single hydrocarbon chain linked to a protein is barely enough to anchor the protein securely, but many proteins have more than one attached lipid moiety. Other interactions, such as ionic attractions between positively charged Lys residues in the protein and negatively charged lipid head groups, probably con tribute to the stability of the attachment. The association of these lipid-linked proteins with the membrane is certainly weaker than that for integral membrane proteins and is, in at least some cases, reversible. But treatment with alkaline carbonate does not release GPI-linked proteins, which are therefore, by the working definition, integral proteins.

Beyond merely anchoring a protein to the mem brane, the attached lipid may have a specific role. In the plasma membrane, proteins with GPI anchors are ex clusively on the outer face and are confined within clusters, as we shall see below, whereas other types of lipid linked proteins (with farnesyl or geranylgeranyl groups attached; Fig. 11–14) are exclusively on the inner face. Attachment of a specific lipid to a newly synthesized membrane protein therefore has a targeting function, directing the protein to its correct membrane location.

FIGURE 11–14 Lipid-linked membrane proteins. Covalently attached lipids anchor membrane proteins to the lipid bilayer. A palmitoyl group is shown attached by thioester linkage to a Cys residue; an N-myristoyl group is generally attached to an amino-terminal Gly; the farnesyl and geranylgeranyl groups attached to carboxyl-terminal Cys residues are isoprenoids of 15 and 20 carbons, respectively. These three lipid-protein assemblies are found only on the inner face of the plasma membrane. Glycosyl phosphatidylinositol (GPI) anchors are derivatives of phosphatidylinositol in which the inositol bears a short oligosaccharide covalently joined to the carboxyl terminal residue of a protein through phosphoethanolamine. GPI-linked proteins are always on the extracellular face of the plasma membrane.

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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:- The Topology of an Integral Membrane Protein Can Be Predicted from Its Sequence

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

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