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-Bungarotoxin Receptors Contain 7 Subunits in Two Different Disulfide-bonded Conformations
Correspondence to: William N. Green, Department of Pharmacological and Physiological Sciences, University of Chicago, 947 East 58th Street, Chicago, IL 60637., wgreen{at}midway.uchicago.edu (E-mail), (773) 702-1763 (phone), (773) 702-3774 (fax)
Neuronal nicotinic 7 subunits assemble into cell-surface complexes that neither function nor bind -bungarotoxin when expressed in tsA201 cells. Functional -bungarotoxin receptors are expressed if the membrane-spanning and cytoplasmic domains of the 7 subunit are replaced by the homologous regions of the serotonin-3 receptor subunit. Bgt-binding surface receptors assembled from chimeric 7/serotonin-3 subunits contain subunits in two different conformations as shown by differences in redox state and other features of the subunits. In contrast, 7 subunit complexes in the same cell line contain subunits in a single conformation. The appearance of a second 7/serotonin-3 subunit conformation coincides with the formation of -bungarotoxin–binding sites and intrasubunit disulfide bonding, apparently within the 7 domain of the 7/serotonin-3 chimera. In cell lines of neuronal origin that produce functional 7 receptors, 7 subunits undergo a conformational change similar to 7/serotonin-3 subunits. 7 subunits, thus, can fold and assemble by two different pathways. Subunits in a single conformation assemble into nonfunctional receptors, or subunits expressed in specialized cells undergo additional processing to produce functional, -bungarotoxin–binding receptors with two 7 conformations. Our results suggest that 7 subunit diversity can be achieved postranslationally and is required for functional homomeric receptors.
Key Words:
-bungarotoxin, 7 subunits, nicotinic receptors, protein folding, disulfide bonds
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