The regulation of acetylcholine receptor expression in mammalian muscle

The synthesis of functional AChRs can be described as a pathway leading from the translation of subunit mRNAs to the plasma membrane forms of extrajunctional and junctional receptors (Fig. 9). We have not included in this scheme pretranslational steps for the synthesis and processing of RNA coding for receptor subunits because very little is known about such processes. Several aspects of Figure 9 are worthy of note: It is now well established that polypeptide synthesis is initiated on free cytoplasmic polysomes and that once sufficient nascent subunits bearing signal peptides at the amino terminus is formed, polysomes assemble with the membranes of the rough endoplasmic reticulum via a mechanism that employs the signal recognition particle (Anderson et al. 1982). Nascent subunits undergo cotranslational insertion through the rough endoplasmic reticulum membrane, signal peptide removal, and core glycosylation (Anderson and Blobel 1981; Merlie et al. 1981; Anderson et al. 1982; Sebbane et al. 1983). Anderson and Blobel (this volume) have demonstrated that subunits synthesized in vitro and inserted into membrane vesicles do not undergo heterologous subunit-subunit associations. We have shown that alpha- and beta-subunits newly synthesized in vivo are not associated with each other. Our data indicate that the alpha-subunit is initially present in vivo in a conformation that is radically different from its native conformation in the mature receptor complex. We assume that beta-, gamma-, and delta-subunits also are synthesized as conformationally "immature" forms, but verification of this point must await the availability of new monoclonal antibody specificities. Our data indicate that only a fraction of the newly synthesized alpha-subunit undergoes conformational maturation to the 5S species which binds both alpha-bungarotoxin and anti-main immunogenic region monoclonal antibodies. alpha-Subunits synthesized during a 5-minute pulse labeling require 30 minutes for completion of this process. alpha-Subunits that do not undergo conformational maturation are degraded rapidly (t1/2 = 0.5 hr) ( Merlie et al. 1982). Assembly of alpha- and beta-subunits synthesized during a 5-minute pulse labeling lags for approximately 30 minutes and is not complete until 90 minutes. Finally, assembled receptors are transported to the surface and appear in the plasma membrane. These processes occur during expression of AChRs in differentiated myoblasts. We do not know how undifferentiated myogenic cells, in vivo or in tissue culture, differ with regard to any of these steps.(ABSTRACT TRUNCATED AT 400 WORDS)