Receptor surface structure

An old hypothesis is based on the observations of Dahlen et al. (D3), who demonstrated that above a certain concentration in plasma, Lp(a) could bind to glycosaminoglycans in the arterial wall (B12). Colocalization of Lp(a) and fibrin on the arterial wall can lead to oxidative changes in the lipid moiety of Lp(a) and induce the formation of oxidatively modified cholesterol esters, which in turn can influence the interaction of Lp(a) and its receptors on macrophages. This process is promoted by the presence of calcium ions. Cushing (C14), Loscalzo (L22), and Rath (R3) reported a colocalization of undegraded Lp(a) and apo-Bl00 in the extracellular space of the arterial wall. In contrast to LDL, Lp(a) is a substrate for tissue transglutaminase and Factor XUIa and can be altered to products that readily interact with cell surface structures (B21). 

Two different systems are involved in the immune response. The innate immune system is based on receptors that can distinguish between bacterial and viral surface structures or foreign proteins (known as antigens) and those that are endogenous. With the help of these receptors, phagocytes bind to the pathogens, absorb them by endocytosis, and break them down. The complement system (see p. 298) is also part of the innate system. 

Early investigations of peptides in membrane model systems included studies of mel-letin 124,125 220 221 spectra and polarization properties. This water-soluble peptide is found to be structureless in solution at neutral pH but was sensitive to environmental change. The undecapeptide hormone, substance P, a member of the tackykinin family, was also found by Choo et a].1222 to be unstructured in solution at physiological pH and to aggregate at high pH or on interaction with charged lipids. These data were used as counter-evidence to a hypothesis that the membrane surface structured the peptide to facilitate interaction with the receptor. 

Tzartos, S. J., Rand, D. E., Einarson, B. L., and Lindstrom, J. M. (1981) Mapping of surface structures of Electrophorus acetylcholine receptor using monoclonal antibodies. J Biol. Chem 256, 8635-8645. 

Note that the mechanism proposed for helper T-cell function requires that T cells have surface receptors that recognize antigen-processing cells, antigen itself, and the appropriate B cell. We will shortly discuss the nature of T-cell receptors, as well as the surface structures they recognize, in a broader context. 

Plakins are not the only proteins that tether IFs to the cell surface. A number of other proteins localized to cell surface structures including desmosomes, focal contacts, and muscle costameres also contribute to IF anchorage at plasma membranes. IFs may also associate with cell surface receptors outside of ultrastructurally distinct structures, a topic that will be dealt with below. 

The physico-chemical parameters of the chemical stimuli which have been shown to have relevance and to be interrelated to the sensory response it elicits as specific odor or taste, are the factors controlling concentration at the receptor areas (solubility, hydrophilicity, lipophilicity, volatility, and partition coefficients), molecular features (size, shape, stereochemical and chirality factors and functional groups), and electronic features (polarity and dipoles) controlling positioning and contact at receptor surfaces (53). Many of these physico-chemical data are not available for many of the chemical stimulants, and till they are gathered, structure-response studies will be much restricted. The effects of interactions of the above parameters appear to a larger degree in the perception of odor, the dimensions of which are many and complex viz. nuances, composite... 

In 1988, Evans observed that organic compounds with certain structures appear to contain common features which facilitate binding to various... receptor surfaces, perhaps through binding elements different from those employed for binding of... 

Holowka D, Baird B. Antigen-mediated IGE receptor aggregation and signaling a window on cell surface structure and dynamics. Annu. Rev. Biophys. Biomol. Struct. 1996 25 79—112. 

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