Membrane proteins anchors

More systematic predictions have been attempted by three groups. First, Cedano et al. (1997) performed a standard discriminant analysis between five classes, each containing 200 examples integral membrane proteins, anchored membrane proteins, extracellular proteins, intracellular proteins, and nuclear proteins. The discrimination was rather clear, except for the distinction between anchored proteins and extracellular/ intracellular proteins. 

Role of PI in membrane protein anchoring Specific proteins can be covalently attached via a carbohydrate bridge to membrane-bound PI (Figure 17.9). [Note Examples of such proteins include alkaline phosphatase (a digestive enzyme found on the surface of the small intestine that attacks organic phosphates), and acetylcholine esterase (an enzyme of the postsynaptic membrane that... 

Example of a phosphatidylinositol glycan membrane protein anchor. GlcNH2 = glucosamine. 

The a and 13 subunits of Gs are water soluble the y subunits, on the other hand are strongly hydrophobic. Since the a/3y complex is hydrophobic, it is currently thought that Gs is a peripheral membrane protein anchored into the-inner leaflet of the membrane bilayer through its y subunit. The possibility exists that, upon activation, a GTP complexes could be released from the membrane. This led Rodbell to postulate functions for such programmable second messengers [33]. 

Tvo additional peripheral membrane proteins anchor the spectrin filaments to the cytoplasmic side of the erythrocyte membrane. One of these polypeptides, the 210-kDa ankyrin protein, binds both a single spectrin molecule and the chloride-bicarbonate anion-exchange protein discussed previously. The second of these polypeptides, actin, is capable of binding several molecules of spectrin. Since actin is able to associate with more than a single spectrin monomer, it acts as a branch point for the spectrin protein as the membrane skeleton or matrix is assembled (see Fig. 13-1). In this experiment, you will determine the concentration of total protein in the erythrocyte membrane through the use of the Folin-Ciocalteau assay. 

Fig. 11.3 Budding model for the biogenesis of intracellular LD. (A) LD formation is initiated by accumulation of TAG between the leaflets of the ER membrane. During budding of the forming LD, typical transmembrane proteins with hydrophilic domains protruding into the ER lumen and cytoplasm are segregated from membrane proteins, anchored in the outer leaflet, only, such as caveolin. After detachment of the matured LD from the ER...

De Jesus AJ, Allen TW (2013) The role of tryptophan side chains in membrane protein anchoring and hydrophobic mismatch. Biochim Biophys Acta 1828(2) 864—876... 

Inositol lipids play specific roles in membrane protein anchoring 353... 

INOSITOL LIPIDS PLAY SPECIFIC ROLES IN MEMBRANE PROTEIN ANCHORING... 

Just how fast can proteins move in a biological membrane Many membrane proteins can move laterally across a membrane at a rate of a few microns per minute. On the other hand, some integral membrane proteins are much more restricted in their lateral movement, with diffusion rates of about 10 nm/sec or even slower. These latter proteins are often found to be anchored to the cytoskeleton (Chapter 17), a complex latticelike structure that maintains the cell s shape and assists in the controlled movement of various substances through the ceil. 

Four different types of lipid-anchoring motifs have been found to date. These are amide-linked myristoyl anchors, thioester-linked fatty acyl anchors, thioether-linked prenyl anchors, and amide-linked glycosyl phosphatidylinosi-tol anchors. Each of these anchoring motifs is used by a variety of membrane proteins, but each nonetheless exhibits a characteristic pattern of structural requirements. 

A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and sometimes to serine or threonine residues within a polypeptide chain (Figure 9.18). This type of fatty acyl chain linkage has a broader fatty acid specificity than A myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the Cjg and Cjg chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein-coupled receptors, the surface glycoproteins of several viruses, and the transferrin receptor protein. 

Protein lateral motion is much slower than that of lipids because proteins are larger than lipids. Also, some membrane proteins can diffuse freely through the membrane, whereas others are bound or anchored to other protein structures in the membrane. The diffusion constant for the membrane protein fibronectin is approximately 0.7 X 10 cmVsec, whereas that for rhodopsin is about 3 X 10 cmVsec. 

FIGURE 15.21 Hormone (H) binding to its receptor (R) creates a hormone receptor complex (H R) that catalyzes GDP-GTP exchange on the o -subunit of the heterotrimer G protein (G ), replacing GDP with GTP. The G -subunit with GTP bound dissociates from the /37-subunits and binds to adenylyl cyclase (AC). AC becomes active upon association with G GTP and catalyzes the formation of cAMP from ATP. With time, the intrinsic GTPase activity of the G -subunit hydrolyzes the bound GTP, forming GDP this leads to dissociation of G GDP from AC, reassociation of G with the /Sy subunits, and cessation of AC activity. AC and the hormone receptor H are integral plasma membrane proteins G and G are membrane-anchored proteins. 

Rieske proteins are constituents of the be complexes that are hydro-quinone-oxidizing multisubunit membrane proteins. All be complexes, that is, bci complexes in mitochondria and bacteria, b f complexes in chloroplasts, and corresponding complexes in menaquinone-oxidizing bacteria, contain three subunits cytochrome b (cytochrome 6e in b f complexes), cytochrome Ci (cytochrome f in b(,f complexes), and the Rieske iron sulfur protein. Cytochrome 6 is a membrane protein, whereas the Rieske protein, cytochrome Ci, and cytochrome f consist of water-soluble catalytic domains that are bound to cytochrome b through a membrane anchor. In Rieske proteins, the membrane anchor can be identified as an N-terminal hydrophobic sequence (13). 

The pecM gene encodes a protein of 297 amino acids with a calculated molecular mass of 32 kDa. The predicted PecM protein displays the characteristics of an integral membrane protein since it is largely hydrophobic, with potential trans-membrane domains. SubceUular firactionation confirmed that PecM is anchored into the bacterial inner membrane whereas PecS is... 

---