Light receptor molecules

The membranes of the rod discs are -60% protein and 40% lipid (Table 8-3). About 80% of the protein is rhodopsin (visual purple), a lipoprotein that is insoluble in water but soluble in detergent solutions. Digito-nin is widely used to disperse rhodopsin molecules because it causes no change in optical properties. In addition to rhodopsin, in the outer segment discs of frog retinal rods, there are -65 molecules of phospholipid and smaller amounts of other materials for each molecule of rhodopsin (Table 8-3). The cone cells have a similar architecture but have a different shape and contain different light receptors. The receptors in the cones are present in deep indentations of the plasma membrane rather than in discs within the cytoplasm. 

Fig. 5.4. Optical Fiber biosensor, (a) Extrinsic optical fiber is used for the guiding the light to and from the sensor area, (b) Intrinsic the receptor molecules are immobilized on the fiber core after decladding of the fiber. The detection is based on fluorescence labels.

Although little is known about the three-dimensional structure of MHC molecules, on the basis of primary sequence data as well as of preliminary crystallographic evidence [96] it can be assumed that their general structural plan is not very different from that of TcR s. Thus, they are composed of two different chains, a heavy (a) and a light (J3) chain, and consist of a membrane-distal domain, to which variability is essentially restricted, a membrane-proximal domain, which is essentially invariant, a transmembrane and an intracytoplasmic tail. Both extracellular domains result from the pairing of two units (Fig. 2) however, in class 1 both V domain units (al and a2) are contributed by the same chain (a), whilst the C domain is contributed by one a chain unit (a3) and by /T-microglobulin (which is not MHC encoded). In class II, both domains result from one a chain and one /3 chain unit (the V domain from al and (31 and the C domain from a2 and (32). Furthermore in class I molecules, but not in class II or T cell receptor molecules, the transmembrane and intracytoplasmic segments are not dimeric. 

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