Membrane microdomains

So long as spatial order of the fluid membrane is maintained, fluidity will actually increase the rate of coupling of independent molecules. It may be worth recalling here that the surface representation of antigen receptors on B and T cells would be very low, were such B and T cell receptors uniformly distributed over the entire ceil surface, and that restriction of their distribution to selected domains of the lymphocyte surface might favor both their clustering by polymeric antigens and their recognition by membranous and cytosolic adaptor and transducer molecules. For instance, assuming it is a smooth sphere, a small B lymphocyte with a mean diameter of 8 |xm would have a surface of about 200 |xm2, but there must be an at least equivalent (if not much higher) amount of surface membrane stored (e.g. for cell division) under the form of various membrane ruffles and pseudopodia or microprojections. Thus, assuming a near-maximal expression of 40 000 mlg molecules for at least 400 ^m2 B cell surface, one reaches the fairly low surface density of si00 mlg molecules per jjim2. While this is a maximal estimate, it is interesting to consider that this corresponds to only one single mlg molecule per patch of 100 nm x 100 nm, which is about the size reported for some membrane microdomains! For comparison, the diameter of the membranous IgM stem (stalk) is in the range of 4 nm only, and the distance between its two Fv domains is about 12 nm. Because of such a low surface density of antigen receptors, cross-link -ing of two individual IgM molecules by an antigen would require much mobility, were they distributed at random.

Similarly, there would be a rather poor efficacy of coupling between recognition by the extracellular antigen/ligand-binding elements, and transduction and signaling by the CD3 complex(-like) elements of the membranous receptor complexes, were they randomly distributed over the whole cell surface. Moreover, the antigen receptor complexes themselves should be in close proximity to their stoichio-metrically associated, membrane-anchored and function-controlling elements such as specific protein kinases and protein phosphatases, as well as to the variety of CD coreceptors which are regulating the signal levels. Therefore, there is a high probability for such receptor-operator units to function within specialized membrane microdomains. The latter might not exist as such in the resting cells, and the fully functional signaling unit would probably proceed through recruitment of some key elements of the receptor-operator unit. Possibly, only a few of these units are functional at any given time, but a first successful coupling of recognitive elements with effector elements might cause something I would call 'membrane stressing'. This might be acting like a catalyst for further unit assembly, e.g. the activation of a phospholipase whose primordial membrane effects would be to favor the further segregation of the membrane into microdomains, which would much enhance the chances of encounters of relevant elements.

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