Cell receptor complexes forming

The interactions between transmitters and their receptors are readily reversible, and the number of transmitter-receptor complexes formed is a direct function of the amount of transmitter in the biophase. The length of time that intact molecules of acetylcholine remain in the biophase is short because acetylcholinesterase, an enzyme that rapidly hydrolyzes acetylcholine, is highly concentrated on the outer surfaces of both the prejunctional (neuronal) and postjunctional (effector cell) membranes. A rapid hydrolysis of acetylcholine by the enzyme results in a lowering of the concentration of free transmitter and a rapid dissociation of the transmitter from its receptors little or no acetylcholine escapes into the circulation. Any acetylcholine that does reach the circulation is immediately inactivated by plasma esterases. 

An important type of receptor is that for low-density lipoprotein (LDL), the principal carrier of cholesterol in the bloodstream. LDL is a particle that consists of various lipids—in particular, cholesterol and phosphoglycerides—as well as a protein. The protein portion of the LDL particle hinds to the LDL receptor of a cell. The complex formed between the LDL and the receptor is pinched off into the cell in a process called endocytosis. (This important aspect of receptor action is described in detail in the articles hy Brown and Goldstein and by Dautry-Varsat and Lodish cited in the bibliography at the end of this chapter.) The receptor protein is then recycled back to the surface of the cell (Figure 8.27). The cholesterol portion of the LDL is used in the cell, but an... 

Class IIHLA molecules are expressed on the surface of antigen-presenting cells. They play a key role in presentation of processed linear peptide antigens of at least nine amino acids to T cells. Antigen is bound to the HLA antigen binding cleft formed by the a and 3 chains of the HLA class II molecule. This tri-molecular HLA-antigen complex binds in turn to the variable portion of the T-cell receptor. 

Fig. 1.—Diagrammatic Representation of the Three Steps in the Taste-cell Transduction. Step 1, interaction of stimulus (S) with membrane-bound receptor (R) to form stimulus-receptor complex (SR) step 2, conformational change (SR) to (SR), brought about by interaction of S with R (this change initiates a change in plasma-membrane conformation of taste cells, probably below the level of the tight junction) and step 3, conformational changes of the membrane result in lowered membrane resistance, and the consequential influx on intracellular ionic species, probably Na. This influx generates the receptor potential which induces synaptic vesicular release to the innervating, sensory nerve, leading to the generator potential.

The cellular mechanism of action of hydrocortisone, a glucocorticoid, is also related to proteins but not by the enhancement of cAMP production. Hydrocortisone is transported by simple diffusion across the membrane of the cell into the cytoplasm and binds to a specific receptor The steroid-receptor complex is activated and enters the nucleus, where it regulates transcription of specific gene sequences into ribonucleic acid (RNA). Eventually, messenger RNA (mRNA) is translated to form specific proteins in the cytoplasm that are involved in the steroid-induced cellular response. 

GABAb receptors are heterodimers. Two GABAb receptor subunits have been cloned, R1 and R2. Neither of these appears to express functional receptors on their own, but they are active when coexpressed, suggesting that a dimer is trafficked to the cell surface and forms an active complex. Evidence shows that the R1 subunit contains the GABA binding site while the R2 subunit interacts with the G protein [14]. 

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