Biogenesis of acetylcholinesterase molecular forms in muscle. Evidence for a rapidly turning over, catalytically inactive precursor pool

Abstract

Tissue-cultured chicken embryo muscle cells synthesize several molecular forms of acetylcholinesterase (AChE) which differ in oligomeric structure and fate as membrane-bound or secreted molecules. Using irreversible inhibitors to inactivate AChE molecules we show that muscle cells rapidly synthesize and assemble catalytically active oligomers which transit an obligatory pathway through the Golgi apparatus. These oligomers acquire complex oligosaccharides and are ultimately localized on the cell surface or secreted into the medium. Immunoprecipitation of isotopically labeled AChE shows that the oligomers are assembled shortly after synthesis from two allelic polypeptide chains. About two-thirds of the newly synthesized molecules are assembled into dimers and tetramers, and once assembled these forms do not interconvert. Comparison of newly synthesized catalytically active AChE molecules with isotopically labeled ones indicates that a large fraction of the immature molecules are catalytically inactive. Pulse-chase studies measuring both catalytic activity and isotopic labeling indicate that only the catalytically active oligomers are further processed by the cell, whereas inactive molecules are rapidly degraded intracellularly by an as yet unknown mechanism. Approximately 70-80% of the newly synthesized AChE molecules are degraded in this manner and do not transit the Golgi apparatus. These studies indicate that muscle cells synthesize an excess of this important synaptic component over that which is necessary for maintaining normal levels of this protein. In addition, these studies indicate the existence of an intracellular route of protein degradation which may function as a post-translational regulatory step in the control of exportable proteins.