Alkylation of Free Sulfhydryls Fortifies Electroplax Subsynaptic Structures

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Abstract
The cysteine‐rich 43,000‐dalton peripheral membrane protein, v1, is localized at the cytoplasmic face of electroplax and muscle cholinergic synapses, where it is thought to play an important role in the endplate supramolecular structure. The peripheral membrane protein properties of v1 are inferred by its removal from nicotinic cholinergic membranes by the action of mild alkali or lithium diiodosa‐licylate. An interesting property of v1 is its high concentration of free sulfhydryl groups, whose exact role in synaptic structure is still largely unknown. Alkylation of free sulfhydryls with N‐ethylmaleimide (3 mM) has a profound effect on the association of v1 with synaptic membranes, rendering v1 unextractable by pH 11 treatment or by lithium diiodosalicylate and, concomitantly, decreasing v1 's electrophoretic mobility on sodium dodecyl sulfate‐polyacrylamide gels. lodoacetamide and iodoacetate have similar effects, but at concentrations 10‐to 100‐fold higher than required for N‐ethylmaleimide. Furthermore, sulfhydryl modification also stabilizes the association of nicotinic receptor subunits with the detergent‐insoluble cytoskeleton. N‐Ethylma‐leimide treatment increases the fraction of insoluble receptor molecules on extraction with Triton X‐100, sodium cholate, or octylglucoside. These results suggest an important role of sulfhydryl groups in the structural stability of the postsynaptic cholinergic membrane.