Biogenesis of the acetylcholine receptor

Reconstitution and bilayer studies provided evidence that the four subunit receptor contains, not only the binding sites for acetylcholine but also the active cation channel, and the activated receptor by itself brings about the response of agonist-induced membrane permeability [50,51]. This is different from the -adrenergic receptor in which the receptor must interact with another membrane protein in order to transmit the signal. 

The biogenesis of both acetylcholine receptor and chromaffin granules share several common properties. The specific polypeptides are synthesized and transported into the membrane by a vectorial translation process. The specific proteins are sorted out by the Golgi apparatus and eventually fuse with the plasma membrane via the secretory pathway. Yet the acetylcholine receptor functions on the plasma membrane, and therefore it should stay on this membrane for a long time (2-7 days). On the other hand, the function of chromaffin granules is to store neurotransmitters. Therefore they stay most of their lifetime inside the cell and their fusion with the plasma membrane is temporary. Soon after the secretion process, the constituents of the chromaffin granule membrane must be removed from the plasma membrane by endocytosis. 

How is the final location of the acetylcholine receptor and secretory vesicles determined Where is the information stored to determine this localization There are two key locations in cells where sorting out of membrane proteins might occur. One of these is the Golgi apparatus and the second one involves the plasma membrane. After chromaffin granules have fused with the plasma membrane, their specific lipids and proteins are immediately and specifically removed from the latter membrane. The mechanism for this selective removal is unknown. In the Golgi apparatus, newly-synthesized membrane proteins are sorted out to various membrane vesicles. We propose a model for the sorting out of secretory vesicles from vesicles destined to deliver plasma membrane proteins. 

Vectorial processing — import of proteins into chloroplasts and mitochondria 

Most of the proteins of chloroplasts and mitochondria are synthesized in the cytoplasm and post-translationally imported into the organelle [29,34], One can divide this process into four steps (1) synthesis of the individual polypeptides as precursors on free ribosomes in the cytoplasm (2) binding of the precursors to specific receptors on the organellar surface (3) transmembrane movement and (4) processing and sorting into the correct compartment. 

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