Hypothetical membrane protein

Figure 1 Modes of diffusion of individual membrane proteins as revealed by single-molecule tracking techniques. The hypothetical trajectory of an individual plasma membrane protein as traced by single-particle tracking techniques is shown. An individual protein can switch between several different modes of over time, which include confined diffusion (region 1), free diffusion (region 2), and immobilization (region 3).

The hypothetical receptor protein is positioned in the cell membrane such that it is sealing an ion channel and contains three binding areas (Fig. 5.12). 

The elucidation of the control mechanism of membrane protein synthesis could be rendered easier if appropriate conditional mutants, such as temperature-sensitive mutants for membrane protein synthesis or regulatory mutants, would be available. Such mutants could prove to be priceless tools if they could be obtained for specific proteins, such as the different components of the L protein group or the hypothetic repressor for the synthesis of these proteins. At the time of this writing, temperature-sensitive mutants for membrane proteins required for photosystem II activity have been obtained and are under investigation (P. Benoun, personal communication). Temperature-sensitive mutants of C. reinhardi for all the cyto-... 

FIG. 17 Schematic illustration of the setup for a tip-dip experiment. First glycerol dialkyl nonitol tetraether lipid (GDNT) monolayers are compressed to the desired surface pressure (measured by a Wilhehny plate system). Subsequently a small patch of the monolayer is clamped by a glass micropipette and the S-layer protein is recrystallized. The lower picture shows the S-layer/GDNT membrane on the tip of the glass micropipette in more detail. The basic circuit for measurement of the electric features of the membrane and the current mediated by a hypothetical ion carrier is shown in the upper part of the schematic drawing. 

Figure 18.16 Hypothetical model for the metallobiology of AP in Alzheimer s disease. (From Bush, 2003. Copyright 2003, with permission from Elsevier.) The proposed sequence of events (1) concentration of iron and copper increase in the cortex with aging. There is an overproduction of APP and AP in an attempt to suppress cellular metal-ion levels. (2) Hyper-metallation of AP occurs which may facilitate H202 production. (3) Hyper-metallated AP reacts with H202 to generate oxidized and cross-linked forms, which are liberated from the membrane. (4) Soluble AP is released from the membrane and is precipitated by zinc which is released from the synaptic vesicles. Oxidized AP is the major component of the plaque deposits. (5) Oxidized AP initiates microglia activation. (6) H202 crosses cellular membranes to react with Cu and Fe, and generate hydroxyl radicals which oxidize a variety of proteins and lipids.

Figure 11.4 The hypothetical pathway for the transformation of a simple RNA cell into a minimal DNA/protein cell. At the first step, the cell contains two ribozymes, Rib-1 and Rib-2 Rib-1 is a RNA replicase capable of reproducing itself and making copies of Rib-2, a ribozyme capable of synthesizing the cell membrane by converting precursor A to surfactant S. During replication, Rib-1 is capable of evolving into novel ribozymes that make the peptide bond (Rib-3) or DNA (Rib-4). In this illustration, these two mutations are assumed to take place in different compartments, which then fuse with each other to yield a protein/DNA minimal cell. Of course, a scheme can be proposed in which both Rib-3 and Rib-4 are generated in the same compartment. (Modified fromLuisi et al., 2002.)...

Fig. 8.9. Crosslinking of signal proteins with the help of protein modnles. A hypothetical protein is shown which contains SH2, SH3, PTB and PH domains. Recognition of phosphotyrosine residues occurs with the help of SH2 or PTB domains SH3 domains bind to proline-rich sequences (Pro in Protein 3) whilst the pleckstrin homology domains (PH domains) mediate binding to phosphatidyl-inositol-phosphates (PtdInsP) in the membrane. In an idealized scheme, the modular protein can associate several proteins (Protein 1 - Protein 3) and mediate interactions between these proteins (shown as broken arrows). The PH domain helps to recruit the complex to the cell membrane favoring interactions with other membrane-associated proteins (Protein X).

FIG. 1.2 A simplified sketch of a hypothetical protein molecule embedded in a bilayer (a biological membrane). The bilayer shown is a two-dimensional cross section of a membrane. The bundle of cylinders shown represents the helices of a protein. The cylinders are part of the same protein and are joined together by other segments (not shown) of the protein protruding out of the bilayer on either side. 

Protons that could logically be involved in a membrane Bohr effect are those present on imidazole rings coordinated to Fe or Cu in redox proteins. Removal of an electron from the metal ion could be accompanied by displacement of electrons within the imidazole, within a peptide group that is hydrogen-bonded to an imidazole, or within some other acidic group. A hypothetical example is illustrated in Eq. 18-12 in which a carboxyl group loses a proton when "handed" a second. If the transiently enolized peptide linkage formed in... 

Figure 2. Hypothetical model showing how a conformational rearrangement of the transmembrane helices may simultaneously change the accessibility of the cation binding site from one side of the membrane to the other and reduce the affinity of the binding site. The open circles represent protein ligands that constitute the binding site. The closed circles represent the ion. Modified from Tanford, 1982a.

Abbreviations [Ca2+]j, intracellular Ca2+ concentration as measured by a Ca2 indicator such as ae-quorin [Ca2+]c, Ca2+ concentration in the bulk cytosol (hypothetical value) [Ca2 f]sm, Ca2 concentration in submembrane domain just beneath the plasma membrane (a hypothetical value) PI, phospha-tidylinositol PIP2, phosphatidylinositol 4,5-bisphosphate PIP, phosphatidylinositol 4-phosphate Insl,4,5,P3, inositol 1,4,5,-trisphosphate Ins 1,3,4,P, inositol 1,3,4-trisphosphate Insl,3,4,5P4, inositol 1,3,4,5-tetrakisphosphate Insl,4P2, inositol 1,4-bisphosphate CaM, calmodulin C-kinase, protein kinase C [cAMP]c, cAMP concentration in the bulk cytosol [cAMP]sm, cAMP concentration in submembrane domain just beneath the plasma membrane. 

Figure 3-14. Hypothetical structures indicating possible mechanisms for transporters and channels in cell membrane (shaded region) (a) mobile carrier or porter acting as a symporter for protons (H+) and some tr ansported solute (5) (b) series of binding sites in a channel across a membrane, acting as a symporter for H+ and S (c) sequential conformations of a channel, leading to unidirectional movement of solute and (d) a protein-lined pore with multiple solute or water molecules hr single file, the most accepted version of ion or water (aquaporirr) channels.

The hydrocarbon core of a membrane is typically 30 A wide, a length that can be traversed by an a helix consisting of 20 residues. We can take the amino acid sequence of a protein and estimate the free-energy change that takes place when a hypothetical a helix formed of residues 1 through 20 is transferred from the membrane interior to water. The same calculation can be made for residues 2 through 21,3 through 22, and so forth, until we reach the end of the sequence. 

Fig. 12.5. Biogenesis and assembly of cytochrome 6-c, complex in the inner mitochondrial membrane. Cytochrome fc-Cj complex contains at least five different subunits COREI (corl), COREII (corll), nonheme iron protein (Fe-S), cytochrome c, (cyt Cj), and cytochrome b (cyt b). Cytochrome f> is a mitochondrial gene product and is probably assembled into the inner membrane (IM) via vectorial translation by mitochondrial ribosomes. The other subunits are synthesized on cytoplasmic ribosomes as larger precursors. The precursors, perhaps in association with a cytoplasmic factor , are attached to receptors on the mitochondrial outer membrane (OM). The complex laterally diffuses to the junctions of the outer and inner membranes, and with the help of a hypothetical translocator the precursors are imported across the membrane. Pre-Corl, pre-Corll, and the pre-nonheme iron protein cross the two membranes, whereas cytochrome c, becomes anchored to the outer face of the inner membrane, facing the intermembrane space (IMS). Cytochrome b is assembled inside the inner membrane, and the nonheme iron protein and Corl and Corll are assembled into the matrix side of the inner membrane. The N-terminal extensions are removed by a soluble matrix protease. The N-terminal extension of cytochrome c, is removed in two steps the first is catalyzed by the matrix protease and the second probably by a protease located on the outer face of the inner membrane.

Figure 17-3 Hypothetical model of a muscarinic receptor showing the location of the transmembrane helical protein domains and the extracellular and Intracellular domains connecting the seven n helical proteins in the membrane. (Reprinted from Goyal. R K. N. Engl. J. Med. 321 1024, 1989, with permission from the author and the Massachusetts Medical Society.)...

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