Proximity and promiscuity

Besides the covalent interactions of membrane elements such as the various polypeptide chains which constitute the TCR, BCR and MHC antigen receptors, and a number of oligomeric CD corecep-tor components which are involved in coupling ligand adhesion to signal transduction, two major mechanisms may favor the selective encounters of membrane components: proximity and promiscuity. The differences between these two terms may look subtle or semantic, but they may be large in terms of mechanisms and consequences.

Promiscuity would refer to membrane components, both integral and peripheral ones, which are loosely held close to each other by low affinity interactions such as those taking place between elements of the 20 nm thick glycocalix (e.g. weak lectin-like interactions in a microenvironment where local carbohydrate concentrations may be in 0.1 m range) and those occurring between the array of membrane-stabilizing proteins constituting the endoskeleton. Moreover, promiscuity may also be created within the membrane by lipid domains showing hydro-phobicity features which may be more or less favorable for the preferential localization of integral proteins with a TM anchor (TM proteins) or with a GPI anchor (GPI proteins), of cytosolic proteins with a myristoyl anchor, and also more or less suitable for the association of some glycolipids, such as ganglio-sides, endowed with an innate tendency for self-association and microdomain formation with the membrane phospholipids. Molecular promiscuity may have a coordinated biosynthetic origin, most interacting elements being synthesized and organized as a membrane patch before the membrane vesicle reaches the cell surface.

Proximity would rather refer to a higher occurrence of membrane components within a micro-domain, which would only be favored by the existence of molecular boundaries in the membrane, in the absence of specific molecular interactions, even of low affinity, between the components of the microdomain. Such domain boundaries might be constituted by protein fences both within the lipid bilayer (integral proteins) and on both of its faces by peripheral proteins, some of which constitute a subplasmalemmal skeleton (membranous endoskeleton) and some others participating to the cell glycocalix (membranous exoskeleton). Within such boundaries of the microdomain, the membrane components would thus be kept in close proximity to each other.

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