Assembly in vivo of mouse muscle acetylcholine receptor: identification of an alpha subunit species that may be an assembly intermediate.

We have studied assembly of acetylcholine receptor in vivo using subunit-specific monoclonal antibodies and immunoprecipitation with alpha-bungarotoxin and antitoxin. We have identified three distinct forms of the alpha subunit. The newly synthesized alpha subunit species has a sedimentation coefficient of 5S and is recognized only by antibody specific for SDS-denatured alpha subunit. We have called this species alpha 61. The 5S alpha Tx species is not associated with beta subunits and is probably monomeric. alpha Tx is formed from alpha 61 with a half-time of 15 min and an efficiency of approximately equal to 30%. Formation of alpha Tx involves a conformational change, and we suggest that this conformation is dependent upon or stabilized by disulfide bond formation. The assembly of alpha Tx with beta subunits (and probably gamma and delta) into a 9S complex appears to be an efficient but slow process requiring more than 90 min. Unassembled alpha 61 subunits are degraded rapidly. However, subunit degradation is a result of failure to assemble, rather than its cause.