Acetylcholine ReceptorfromMammalian MuscleIdentifies theMembrane-spanningRegionsinthea and SSubunits

We haveinvestigated thetopologyofthea and S subunitsofthenicotinicacetylcholinereceptor (AChR) from mammalian musclesynthesizedinan invi- trotranslation systemsupplementedwithdog pancreatic microsomes.Fusion proteinswere expressedinwhich a carboxy-terminalfragmentofbovineprolactinwas attacheddownstream ofeachofthemajor putative transmembrane domains,Ml-M4 and MA, intheAChR subunits.The orientationoftheprolactindomain rela- tivetothemicrosomal membrane was thendetermined foreachproteinby aproteolysisprotectionassay.Since theprolactindomain containsno informationwhich eitherdirectsorpreventsitstranslocation, itstrans-