Transmembrane topology of acetylcholine receptor subunits probed with photoreactive phospholipids

The domains of the acetylcholine receptor subunits that contact the lipid phase were investigated by hydrophobic photolabeling of receptor-rich membrane fragments prepared from Torpedo marmorata and Torpedo californica electric organs. The radioactive arylazido phospholipids used carry a photoreactive group, either at the level of the lipid polar head group (PCI) or at the tip of the aliphatic chain (PCII), and thus probe respectively the "superficial" and "deep" regions of the lipid bilayer. The four subunits of T. marmorata and T. californica acetylcholine receptor reacted with both the PCI and PCII probes and thus are all exposed to the lipid phase. Ligands known to stabilize different conformations of the acetylcholine receptor (nicotinic agonists, snake alpha-toxin, and noncompetitive blockers) did not cause any significant change in the labeling pattern. The acetylcholine receptor associated 43 000-dalton v1 protein did not react with any of the probes. A striking difference in labeling between T. marmorata and T. californica acetylcholine receptors occurred at the level of the alpha-subunit when the superficial PCI probe was used. An approximately 5-fold higher labeling of the alpha-subunit as compared to the beta-, gamma-, and delta-subunits was observed by using receptor-rich membranes from T. marmorata but not from T. californica. The same difference persisted after purification of the labeled receptors from the two species and was restricted to an 8000-dalton C-terminal tryptic peptide. The only mutation observed in this region of the complete alpha-subunit sequence of the two species is the substitution of cysteine-424 in T. marmorata by serine-424 in T. californica.(ABSTRACT TRUNCATED AT 250 WORDS)