Structural characterization of alpha-bungarotoxin-binding proteins from Aplysia californica

Structural features of alpha-bungarotoxin-binding proteins from the marine mollusc Aplysia californica have been examined as a first step toward delineating their potential role in cholinergic neurotransmission. Protein blotting with 125I-alpha-bungarotoxin was used to identify binding proteins in membranes prepared from Aplysia muscle and nervous tissue. Binding proteins from both tissues exhibited similar physical characteristics, which distinguish them from the prototypical alpha-bungarotoxin-binding protein, the nicotinic acetylcholine receptor obtained from Torpedo californica electric organ membranes. Aplysia binding activities migrate with an apparent molecular weight of 250 kDa on sodium dodecyl sulfate (SDS) gels in the presence of reducing agents. Binding of alpha-bungarotoxin to blots of Aplysia membranes is abolished by exposure of samples to heat or to low pH but is unaffected by reduction-alkylation treatment. In contrast, the alpha-bungarotoxin-binding subunit of the acetylcholine receptor from Torpedo membranes migrates on SDS gels at 40 kDa. It retains binding activity following exposure to heat or to low pH, but binding is substantially diminished by reduction-alkylation treatments. Another distinguishing characteristic of the Aplysia binding activities is revealed by examining recovery of membrane alpha-bungarotoxin-binding on protein blots; the high recovery of Aplysia binding contrasts sharply with the low recovery of Torpedo binding activity. The high apparent molecular weight of the Aplysia alpha-bungarotoxin-binding activities, their most distinguishing feature, is similar to an alpha-bungarotoxin-binding activity recently identified in lower vertebrate brain. Covalent cross-linking with 125I-alpha-bungarotoxin demonstrates, however, that the mobility of both Aplysia binding activities is due to a multimeric protein that is unusually resistant to dissociation in SDS. The covalently radiolabeled Aplysia alpha-bungarotoxin-binding activity migrates at approximately 260 kDa on SDS gels when solubilized at room temperature. When it was boiled before electrophoresis, the mobility of the radiolabeled protein shifts to approximately 70 kDa. Resistance to dissociation in the absence of boiling may explain both the high recovery of activity on blots and the insensitivity to reductive alkylation. Conversely, dissociation of the multimeric complex upon boiling may explain the observed loss of binding activity. Our results demonstrate structural similarities and differences between Aplysia alpha-bungarotoxin-binding proteins and the Torpedo acetylcholine receptor.(ABSTRACT TRUNCATED AT 400 WORDS)