Identification of homo-oligomers as potential intermediates in acetylcholine receptor subunit assembly

We have examined the sedimentation behavior, on sucrose density gradients, of acetylcholine receptor (AcChoR) subunits synthesized in vitro and integrated into heterologous rough microsomal membranes. In media containing nondenaturing detergents such as Triton X-100 or deoxycholate, the subunits appear to self-associate although, as previously reported, no heterologous interactions were detected. The sedimentation profiles assume a broad distribution in the region of 7-13 S. However, the peak fractions occupy the same region of the gradient as does native AcChoR, run in parallel. Such large homo-oligomers were not observed for another membrane protein, opsin, studied in the same way. This indicated that the associations are indeed between the AcChoR subunits and not simply between all newly synthesized membrane proteins. The homologous associations are interpreted to suggest a mechanism for maintaining the ionophore surfaces of the subunits in an energetically preferred, but metastable, configuration during the lengthy period of post-translational assembly.