Membranes cell interactions with

Further progress may derive from a more accurate definition of the chemical and physical properties of the humic substances present at the rhizosphere and how they interact with the root-cell apoplast and the plasma membrane. An interaction with the plasma membrane H -ATPase has already been observed however this master enzyme may not be the sole molecular target of humic compounds. Both lipids and proteins (e.g., carriers) could be involved in the regulation of ion uptake. It therefore seems necessary to investigate the action of humic compounds with molecular approaches in order to understand the regulatory aspects of the process and therefore estimate the importance of these molecules as modulators of the root-soil interaction. 

Maurel, D., Kniazeff, J., Mathis, G., Trinquet, E., Pin, J. P. and Ansanay, FI. (2004). Cell surface detection of membrane protein interaction with homogeneous time-resolved fluorescence resonance energy transfer technology. Anal. Biochem. 329, 253-62. 

Membranes separate cells from their external environment, and the internal components of cells from each other. Many biochemical processes taking place within cells occur on a framework of membranes. Toxicant interactions with membranes figure prominently in many types of toxic effect. 

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).

B lymphocytes may be activated by a cell-cell interaction with T lymphocytes, i.e., they are stimulated to proliferate and produce antibodies. Stimulation of B lymphocytes takes place in a complex with T lymphocytes and this complex formation is mediated by a number of protein-protein interactions of membrane proteins from both cell types. The proteins involved are receptor systems with corresponding hgands on the partner cell. Tlie hgands are either secreted proteins or membrane proteins that speci-ficaUy bind to receptors on the surface of the partner cell, which in this case is a B or T lymphocyte. 

Selectins are a family of plasma membrane lectins that mediate cell-cell recognition and adhesion in a wide range of cellular processes. One such process is the movement of immune cells (T lymphocytes) through the capillary wall, from blood to tissues, at sites of infection or inflammation (Fig. 7-33). At an infection site, P-selectin on the surface of capillary endothelial cells interacts with a specific oligosaccharide of the glycoproteins of circu-... 

Saponins consist of a terpenoid core (the aglycone), having oxygenated positions bound to sugar moieties (up to ten monosaccharidic units). In water they form colloidal solutions which foam on shaking and precipitate cholesterol. When saponins are near cell membranes, their interaction with cholesterol may create pore-like structures that eventually cause the membrane to burst. Hemolysis is an example of this phenomenon (i.e. the distraction of erythocyte membranes, but not hemoglobin). Occasionally, they cause hypersecretion, which could explain their expectorant activities and also their toxicity to fish. 

We have been interested in the contribution made by plasma GSL towards cell membrane-lipoprotein interaction with regard to ... 

GSLs play crucial roles in functions of the nervous system and skin, cell growth and differentiation, infections, cancer, and immune response [1, 2, 12], Owing to their strategic position in membranes, they interact with toxins, bacteria, and viruses. They form membrane lipid rafts and present the attached carbohydrates as cell-surface receptors and, thus, serve as portals of entry for pathogens through carbohydrate-protein interactions [13]. For example, HIV entry is mediated by GalCer receptors of the host cells [14],... 

Figure 16.15 Effect of triiodothyronine on cells. In the bloodstream, protein-bound triiodothyronine (P-T3) is in equilibrium with free T3. The latter penetrates cell membranes with ease and may be concentrated in the cytosol by a T3 binding protein (Rc). Free cytosolic T3, in equilibrium with Rc-T3, may interact with mitochondrial T3 receptors or may penetrate the nuclear membrane to interact with a T3 receptor on chromatin.

For any chemical messenger to elicit an effect it must first be recognised by the cell. Peptide messengers interact with cell-surface receptors (peptides do not cross the membrane), whereas steroid- and lipid-derived messengers diffuse across the cell membrane and interact with intracellular receptors. Ligand-gated ion-channel receptors are a class of receptor that may occur either at the cell surface or within the cell. 

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