Association with membranes

Carriers and channels may be distinguished on the basis of their temperature dependence. Channels are comparatively insensitive to membrane phase transitions and show only a slight dependence of transport rate on temperature. Mobile carriers, on the other hand, function efficiently above a membrane phase transition, but only poorly below it. Consequently, mobile carrier systems often show dramatic increases in transport rate as the system is heated through its phase transition. Figure 10.39 displays the structures of several of these interesting molecules. As might be anticipated from the variety of structures represented here, these molecules associate with membranes and facilitate transport by different means. 

Figure 3. Electron micrographs of myelinated axons of Xenopus laevis. Upper figure Cross section of axon showing microtubules in groups in association with membrane-bound organelles. Lower figure Longitudinal section of axon showing neurofilaments and microtubules in close proximity to membrane-bound organelles. (Courtesy of Dr. R. Smith.)...

Figure 43-13. Several signal transduction pathways converge on CBP/p300. Ligands that associate with membrane or nuclear receptors eventually converge on CBP/p300. Several different signal transduction pathways are employed. EGF, epidermal growth factor GH, growth hormone PrI, prolactin TNF, tumor necrosis factor other abbreviations are expanded in the text.

UDP-GTase may be associated with membrane transporters and is a key component of pigment accumulation for the metabolic engineer to consider. 

Carotenoids are also present in animals, including humans, where they are selectively absorbed from diet (Furr and Clark 1997). Because of their hydrophobic nature, carotenoids are located either in the lipid bilayer portion of membranes or form complexes with specific proteins, usually associated with membranes. In animals and humans, dietary carotenoids are transported in blood plasma as complexes with lipoproteins (Krinsky et al. 1958, Tso 1981) and accumulate in various organs and tissues (Parker 1989, Kaplan et al. 1990, Tanumihardjo et al. 1990, Schmitz et al. 1991, Khachik et al. 1998, Hata et al. 2000). The highest concentration of carotenoids can be found in the eye retina of primates. In the retina of the human eye, where two dipolar carotenoids, lutein and zeaxan-thin, selectively accumulate from blood plasma, this concentration can reach as high as 0.1-1.0mM (Snodderly et al. 1984, Landrum et al. 1999). It has been shown that in the retina, carotenoids are associated with lipid bilayer membranes (Sommerburg et al. 1999, Rapp et al. 2000) although, some macular carotenoids may be connected to specific membrane-bound proteins (Bernstein et al. 1997, Bhosale et al. 2004). 

Inositol triphosphate (IP3)-gated channels are also associated with membrane-bound receptors for hormones and neurotransmitters. In this case, binding of a given substance to its receptor causes activation of another membrane-bound protein, phospholipase C. This enzyme promotes hydrolysis of phosphatidylinositol 4,5-diphosphate (PIP2) to IP3. The IP3 then diffuses to the sarcoplasmic reticulum and opens its calcium channels to release Ca++ ions from this intracellular storage site. 

EP0445896 26 A membrane fragments extract from R. rhodochrous strain ATCC No. 53968 (and/or B.sphaericus strain ATCC No. 53969) Extract and/or enzymes Enzymes associated with membrane [52]... 

Kadurugamuwa JL, Beveridge TJ (1995) Virulence factors are released from Pseudomonas-aeruginosa in association with membrane-vesicles during normal growth and exposure to gentamicin - a novel mechanism of enzyme-secretion. J Bacteriol 177 3998-4008... 

Membrane depolarization typically results from an increase in Na+ conductance. In addition, mobilization of intracellular Ca2+ from the endoplasmic or sarcoplasmic reticulum and the influx of extracellular Ca2+ appear to be elicited by ACh acting on muscarinic receptors (see Ch. 22). The resulting increase in intracellular free Ca2+ is involved in activation of contractile, metabolic and secretory events. Stimulation of muscarinic receptors has been linked to changes in cyclic nucleotide concentrations. Reductions in cAMP concentrations and increases in cGMP concentrations are typical responses (see Ch. 21). These cyclic nucleotides may facilitate contraction or relaxation, depending on the particular tissue. Inhibitory responses also are associated with membrane hyperpolarization, and this is a consequence of an increased K+ conductance. Increases in K+ conductance may be mediated by a direct receptor linkage to a K+ channel or by increases in intracellular Ca2+, which in turn activate K+ channels. Mechanisms by which muscarinic receptors couple to multiple cellular responses are considered later. 

Rab is a family of small G proteins involved in membrane vesicle trafficking. Mammalian tissues contain around 30 forms of Rab, which specifically associate with the various types of membrane vesicles and organelles that exist in cells [30,31]. Rab proteins, named originally as ras-related proteins in brain, are isoprenylated and associate with membranes, as do isoprenylated Ras and G protein y subunits. However, unlike these other G proteins, the GTP and GDP binding to Rab appears to regulate its association with membrane compartments. 

The y and 8 forms of PLC can be distinguished from PLC(3 by the virtual absence of a 400-500 amino acid consensus sequence present in the C-terminal region of PLC(3. Consistent with the absence of putative transmembrane spanning domains, most PLC activity is localized to the cytoplasm, although a significant amount of activity is associated with membrane fractions. 

Transport of MBOs to their various destinations is typically mediated by MTs and motor molecules. Membrane and secretory proteins become associated with membranes either during or immediately following their synthesis, and then maintain this association throughout their lifetime in the cell. For example, inhibiting synthesis of either protein or phospholipid leads to a proportional decrease in the amount of both protein and phospholipid... 

Quantitative studies comparing the relative abundances of proteins in different cellular states may be performed with MS. Methods such as 2D-GE have been utilized extensively with great success and differentially represent spots excised and then subjected to MS/MS for final identification of the differentially expressed proteins. 2D-GE requires approximately 50 pg of starting material and is limited by its bias toward high abundance proteins and propensity to detect proteins with extreme pi values. Furthermore, proteins at both extremes of molecular weight and those associated with membrane fractions are not well represented by 2D-GE.10... 

The enzyme responsible for this topping-up ATP in active muscle is CK. CK is found in high concentration in muscle cells, both free within the sarcoplasm and also associated with membranes of mitochondria and the sarcoplasmic reticulum. Structurally, creatine kinase is a dimeric enzyme of B and/or M subunits, each of about 40 kDa. Three quaternary structure isoenzyme forms arise CK-MM, CK-BB and CK-MB. The predominant form in all muscles is CK-MM, but cardiac muscle also contains a significant amount of CK-MB and this isoenzyme can be used as a specific marker of myocardial damage (see Case Notes at the end of this chapter). 

Other DUBs have been found to associate with membranes and regulate membrane-associated cellular processes, although they appear not to be membrane anchored like UBP16. The ability of DOA4 to remove ubiquitin from membrane-bound endocytic substrates promotes their degradation in the vacuole or lysosome [71]. DUBs are also important for membrane fusion events as shown by the fact that an OTU domain DUB, VCIP135 (VCP/p47 complex-interacting protein of... 

Biain adenosinetriphosphatase This enzymatic activity is persistendy associated with brain micFotubules even after multiple cycles of warm-induced microtubule assembly, centrifugation to separate protomer and polymer, cold-induced disassembly, and subsequent centrifugation to remove cold-stable aggregates (White et aL, 1980). The enzyme hydrolyzes boA GTP and ATP, and recent work by Tominaga and Kaziro (1982) indicates that there are two distinct ATP-ases, one that is of low M, (around 33,000) and tubulin dependent in the presence of calcium ion, and the other of larger size and associated with membrane vesicles... 

In contrast, myosin I, which is not present in muscle, possesses only one head region and a short tail. Its role in cells may be involved in movement associated with membranes (endocytosis, phagocytosis). 

Heme dissociates from methemoglobin or metmyoglobin in the circulation and can be boimd by hemopexin or albumin, a heme binding plasma protein of lower avidity than hemopexin (49). It is important that the heme be controlled, since this amphipathic, oxidatively active compound can nonspecifically associate with membrane lipids or lipoproteins and cause oxidative damage of vital biomolecules, including DNA (50, 51). 

Lipid Metabolism. Next we explored changes in lipid metabolism in leaves exposed to ozone. Sterols and sterol derivatives were particularly interesting to us because they have been associated with membrane-containing fractions of leaves ( ). Changes produced in these compounds may be early events in the toxicity of ozone to plant cells. 

Figure 4-4. The domain organization of an integral, transmembrane protein as well as the mechanisms for interaction of proteins with membranes. The numbers illustrate the various ways by which proteins can associate with membranes I, multiple transmembrane domains formed of a-helices 2, a pore-forming structure composed of multiple transmembrane domains 3, a transmembrane protein with a single a-helical membrane-spanning domain 4, a protein bound to the membrane by insertion into the bilayer of a covalently attached fatty acid (from the inside) or 5, a glycosyl phosphatidylinositol anchor (from the outside) 6, a protein composed only of an extracellular domain and a membrane-embedded nonpolar tail 7, a peripheral membrane protein noncova-lently bound to an integral membrane protein.

Among the earliest proposals to explain the mechanism of action of anesthetics is the concept that they interact physically rather than chemically with lipophilic membrane components to cause neuronal failure. However, this concept proposes that all anesthetics interact in a common way (the unitary theory of anesthesia), and it is being challenged by more recent work demonstrating that specific anesthetics exhibit selective and distinct interactions with neuronal processes and that those interactions are not easily explained by a common physical association with membrane components. Proposals for the production of anesthesia are described next. 

In living cells, water is the predominant solvent. It is therefore not surprising that scientific studies of enzymes have been carried out mainly in aqueous media. Often qnite dilute solutions of substrates and enzymes in aqueous buffers have been studied. However, one should bear in mind that high concentrations of proteins, other biopolymers and low molecular weight compounds are present around the enzymes in living cells. Furthermore, some enzymes are associated with membrane stmctures containing mainly hydrophobic lipids. Accordingly, some of the non-conventional ... 

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