Synthesis, transport and fate of acetylcholinesterase in cultured chick embryos muscle cells.

We have found that approximately one third of the total cell-associated acetylcholinesterase (AChE) is located on the plasma membrane of cultured chick embryo muscle, the remaining two thirds being found within the cells. This cell surface AChE appears to be an integral membrane protein. The surface enzyme is synthesized by the muscle cells in culture and is transported over a 2-3 hr period to the plasma membrane, where it accumulates at the rate of 2-3% of total surface AChE per hour. Once on the plasma membrane the AChE molecules are degraded by a process that exhibits first-order decay kinetics with a half-life of about 50 hr. Under the same experimental conditions, the acetylcholine receptor, a well described muscle cell integral membrane protein, has a half-life of approximately 19 hr. These studies provide the first direct evidence that the numbers of different muscle plasma membrane glycoprotein molecules are determined not only by differential rates of biosynthesis but also by differential rates of degradation. The intracellular AChE constitutes a rapidly turning-over pool of molecules. The rate of synthesis of AChE in culture is approximately 20% of the total cell-associated enzyme per hour, most of which is destined for secretion into the medium. Only a small portion of the newly synthesized AChE is retained on the plasma membrane. The time from synthesis to release of the enzyme is 2-3 hr. Using 3H-DFP to label the newly synthesized AChE, we can also show a quantitative transfer of AChE molecules from the intracellular to the extracellular compartments without any detectable residence time on the plasma membrane. By studying the synthesis transport and externalization of AChE we have defined the intracellular transport pathway and metabolic requirements for secretion in cultured muscle cells. These studies form the basis for a comparison of the metabolism of membrane-bound and secreted glycoproteins from this cell type.