Proteins membrane-associated

Proximity imaging by energy transfer Protein-protein binding Protein-membrane association... 

Fig. 9.6. Lipid anchor of the Ras protein. Membrane association of the Ras protein is mediated via a palmitoyl and a farnesyl anchor (see also 3.7).

Xu J, Paquet M, Lau AG, Wood JD, Ross CA, Hall RA. Beta 1-adrenergic receptor association with the synaptic scaffolding protein membrane-associated guany-late kinase inverted-2 (MAGI-2). Differential regulation of receptor internalization by MAGI-2 and PSD-95. J Biol Chem 2001 276 41,310-41,317. 

Protein-membrane association via a post-translational modification introduces the notion of dynamic association and partitioning of proteins between the membrane phase of the cells and the aqueous phase (cytosol or inner phase of organelles). Consequently, such proteins can be found both as membrane-associated and membrane-free, which is not the case with intrinsic membrane proteins which are strictly membrane embedded. Another type of association to membrane is mediated by protein-protein interactions with other membrane proteins. A typical example of this situation is provided by the respiratory complexes. In the case of ubiquinol-cytochrome c oxidoreductase, core proteins 1 and 2 does not show any interaction with the lipid membrane, but only with the protein subunits spanning the membrane (e.g. cytochrome b) (Iwata et al. 1998). 

Dormann, D., Libotte, T, Weijer, C. J., and Bretschneider, T. (2002) Simultaneous quantification of cell motility and protein-membrane-association using active contours. Cell Motil. Cytoskeleton 52, 221-230... 

The polypeptide chain of Src tyrosine kinase, and related family members, comprises an N-terminal "unique" region, which directs membrane association and other as yet unknown functions, followed by a SH3 domain, a SH2 domain, and the two lobes of the protein kinase. Members of this family can be phosphorylated at two important tyrosine residues—one in the "activation loop" of the kinase domain (Tyr 419 in c-Src), the other in a short... 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

The hormonal stimulation of adenylyl cyclase is effected by a transmembrane signaling pathway consisting of three components, all membrane-associated. Binding of hormone to the external surface of a hormone receptor causes a conformational change in this transmembrane protein, which in turn stimulates a GTP-binding protein (abbreviated G protein). G proteins are heterotrimeric proteins consisting of a- (45-47 kD), /3- (35 kD), and y- (7-9 kD) subunits. The a-subunit binds GDP or GTP and has an intrinsic, slow... 

All these intermediates except for cytochrome c are membrane-associated (either in the mitochondrial inner membrane of eukaryotes or in the plasma membrane of prokaryotes). All three types of proteins involved in this chain— flavoproteins, cytochromes, and iron-sulfur proteins—possess electron-transferring prosthetic groups. 

What molecular architecture couples the absorption of light energy to rapid electron-transfer events, in turn coupling these e transfers to proton translocations so that ATP synthesis is possible Part of the answer to this question lies in the membrane-associated nature of the photosystems. Membrane proteins have been difficult to study due to their insolubility in the usual aqueous solvents employed in protein biochemistry. A major breakthrough occurred in 1984 when Johann Deisenhofer, Hartmut Michel, and Robert Huber reported the first X-ray crystallographic analysis of a membrane protein. To the great benefit of photosynthesis research, this protein was the reaction center from the photosynthetic purple bacterium Rhodopseudomonas viridis. This research earned these three scientists the 1984 Nobel Prize in chemistry. 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

PH domains consist of about 120 amino acid residues. They do not interact with other proteins, but associate with specific polyphosphoinositides. Consequently, PH domains appear to be important for localizing target proteins to the plasma membrane. Examples of PH domain-containing proteins include phospholipase C andpl20/RasGAP (Fig. 1). 

In the case of L-type Ca2+ channels, they also carry binding sites for Ca2+ antagonist drugs. The accessory a2-5, p, and y subunits stabilize Ca2+ channel function and support its targeting to the plasma membrane. Notably other proteins can associate with the channel complex allowing the formation of signaling complex important for channel targeting and modulation. 

DMD and BMD DMD and BMD are caused by the absence or deficiency of dystrophin a membrane-associated protein, resulting in increased Ca2+ concentration in muscle, loss of Ca2+ homeostasis, and inappropriate calpain activity36... 

Semaphorins are secreted, membrane-associated or transmembrane proteins defined by the presence of a sema-phorin protein domain (Serna domain). In the mammalian system, more than 20 semaphorins have been identified which play important roles in a variety of tissues. The best characterized receptors for mediating semaphoiin effects are members of the neuropilin and plexin families of transmembrane proteins. Semaphoiin functions are best described in the regulation of neural development, angiogenesis, immunoregulation and cancer. 

The VACM-1 receptor is a membrane-associated protein with a single putative transmembrane domain that binds selectively AVP (XD — 2 nM), but cannot discriminate between VXR and V2R analogues. It is expressed in endothelial and medullary collecting duct cells and upon stimulation by AVP. It induces a mobilization of cytosolic-free Ca2+, decreases cAMP production and inhibits cellular growth via MAPK phosphorylation and p53 expression. The mechanism of action and physiological functions of this new receptor are not well understood, but it seems to participate in the regulation of AVP induced signal transduction pathways or of a yet unidentified peptide. 

Figured. Diagrammatic representation of the red blood cell cytoskeletal-plasma membrane complex. Spectrin is made up of many homologous triple-helical segments joined by nonhelical regions (Speicher and Marchesi, 1984). Spectrin and actin require accessory proteins to form a membrane-associated network. (This diagram is constructed from data previously published for example, see Stryer, 1988 Davies and Lux, 1989 Bennett and Gilligan, 1993).

Muscle contraction is initiated by a signal from a motor nerve. This triggers an action potential, which is propagated along the muscle plasma membrane to the T-tubule system and the sarcotubular reticulum, where a sudden large electrically excited release of Ca " into the cytosol occurs. Accessory proteins closely associated with actin (troponins T, I, and C) together with tropomyosin mediate the Ca -dependent motor command within the sarcomere. Other accessory proteins (titin, nebulin, myomesin, etc.) serve to provide the myofibril with both stability... 

There is a substantial weight of evidence for the cytoskeleton being responsible for the force production and control of cell locomotion. This view has not yet been accepted unanimously. However, an alternative hypothesis continues to be argued which states that membrane cycling is the motive force driving cell locomotion (Bretscher, 1987). One of the predictions of the membrane flow hypothesis is that there should be a discernible flow of lipid from the front to the rear of the cell. Lipid flow has proven very difficult to study, because of the lack of suitable methods to label single lipid molecules and the heterogenous behavior of membrane-associated proteins. The observation that particles were transported rearward when they bound... 

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