Abstract
In addition to actions mediated by changes in gene expression, steroids can directly modulate several transmitter-gated and voltage-gated ion channels. Despite numerous studies showing that steroids enhance or reduce ion channel activity, the site(s) that mediates steroid recognition is not known. To identify the regions in which steroids bind and affect ion channel activity, we have taken advantage of the observation that human alpha4beta2 neuronal nicotinic receptors are potentiated by an estrogen steroid, 17beta-estradiol, whereas a rat alpha4beta2 receptor is not. Mutations indicate that a sequence (AGMI) at the end of the C terminus of the human alpha4 subunit forms a binding site required for 17beta-estradiol potentiation. In contrast, ethynyl beta-estradiol (an oral contraceptive) potentiates both human and rat alpha4beta2 receptors. A single tryptophan in the C terminus of both the rat and the human alpha4 subunit is sufficient for potentiation by ethynyl beta-estradiol, probably through a pi-pi interaction. Mutation of this tryptophan eliminates the ability of ethynyl beta-estradiol to potentiate rat receptors. However, in human receptors it was necessary to mutate both the AGMI sequence and the tryptophan to eliminate potentiation by ethynyl beta-estradiol. The findings that beta-estradiol requires the AGMI sequence but that a single C-terminal tryptophan is sufficient for potentiation by ethynyl beta-estradiol indicate that the C terminus forms a binding site for these steroids. The binding site(s) for block appears to differ from those involved in potentiation because the C-terminal sequence does not affect block by steroids such as progesterone, and progesterone does not competitively inhibit potentiation.