NMR Structure and Action on Nicotinic Acetylcholine Receptors of Water-soluble Domain of Human LYNX1

Discovery of proteins expressed in the central nervous system sharing the three-finger structure with snake alpha-neurotoxins provoked much interest to their role in brain functions. Prototoxin LYNX1, having homology both to Ly6 proteins and three-finger neurotoxins, is the first identified member of this family membrane-tethered by a GPI anchor, which considerably complicates in vitro studies. We report for the first time the NMR spatial structure for the water-soluble domain of human LYNX1 lacking a GPI anchor (ws-LYNX1) and its concentration-dependent activity on nicotinic acetylcholine receptors (nAChRs). At 5-30 mu M, ws-LYNX1 competed with I-125-alpha-bungarotoxin for binding to the acetylcholine-binding proteins (AChBPs) and to Torpedo nAChR. Exposure of Xenopus oocytes expressing alpha 7 nAChRs to 1 mu M ws-LYNX1 enhanced the response to acetylcholine, but no effect was detected on alpha 4 beta 2 and alpha 3 beta 2 nAChRs. Increasing ws-LYNX1 concentration to 10 mu M caused a modest inhibition of these three nAChR subtypes. A common feature for ws-LYNX1 and LYNX1 is a decrease of nAChR sensitivity to high concentrations of acetylcholine. NMR and functional analysis both demonstrate that ws-LYNX1 is an appropriate model to shed light on the mechanism of LYNX1 action. Computer modeling, based on ws-LYNX1 NMR structure and AChBP x-ray structure, revealed a possible mode of ws-LYNX1 binding.