The myristoylated protein rapsyn is cotargeted with the nicotinic acetylcholine receptor to the postsynaptic membrane via the exocytic pathway

Rapsyn, a 43 kDa protein required to cluster nicotinic acetylcholine recept ors (AChRs) at the neuromuscular junction, is tightly associated with the p ostsynaptic membrane via an N-terminal myristoylated site. Recent studies h ave shown that some acylated proteins associate with the exocytic pathway t o become targeted to their correct destination. In this work, we used Torpe do electrocyte to investigate the intracellular routing of rapsyn compared to those of AChR and Na, K-ATPase, the respective components of the innerva ted and noninnervated membranes. We previously demonstrated that these latt er two proteins are sorted and targeted to plasma membrane via distinct pop ulations of post-Golgi vesicles (Camus et al., 1998). Biochemical and immun oelectron microscopy analyses of various populations of post-Golgi vesicles immunopurified with magnetic beads led us to identify post-Golgi transport vesicles containing both rapsyn and AChR. These data suggest that rapsyn, as for AChR, specifically follows the exocytic pathway. Furthermore, immuno gold-labeling experiments provided in situ evidence that AChR and rapsyn ar e cotransported in the same post-Golgi vesicles. Taken together, our observ ations suggest that rapsyn and AChR are cotargeted to the postsynaptic memb rane.