Three‐finger proteins from snakes and humans acting on nicotinic receptors: Old and new

The first toxin to give rise to the three‐finger protein (TFP) family was α‐bungarotoxin (α‐Bgt) from Bungarus multicinctus krait venom. α‐Bgt was crucial for research on nicotinic acetylcholine receptors (nAChRs), and in this Review article we focus on present data for snake venom TFPs and those of the Ly6/uPAR family from mammalians (membrane‐bound Lynx1 and secreted SLURP‐1) interacting with nAChRs. Recently isolated from Bungarus candidus venom, αδ‐bungarotoxins differ from α‐Bgt: they bind more reversibly and distinguish two binding sites in Torpedo californica nAChR. Naja kaouthia α‐cobratoxin, classical blocker of nAChRs, was shown to inhibit certain GABA‐A receptor subtypes, whereas α‐cobratoxin dimer with 2 intermolecular disulfides has a novel type of 3D structure. Non‐conventional toxin WTX has additional 5th disulfide not in the central loop, as α‐Bgt, but in the N‐terminal loop, like all Ly6/uPAR proteins, and inhibits α7 and Torpedo nAChRs. A water‐soluble form of Lynx1, ws‐Lynx1, was expressed in E. coli, its 1H‐NMR structure and binding to several nAChRs determined. For SLURP‐1, similar information was obtained with its recombinant analogue rSLURP‐1. A common feature of ws‐Lynx1, rSLURP‐1, and WTX is their activity against nAChRs and muscarinic acetylcholine receptors. Synthetic SLURP‐1, identical to the natural protein, demonstrated some differences from rSLURP‐1 in distinguishing nAChR subtypes. The loop II fragment of the Lynx1 was synthesized having the same µM affinity for the Torpedo nAChR as ws‐Lynx1. This review illustrates the productivity of parallel research of nAChR interactions with the two TFP groups. Nicotinic acetylcholine receptors (nAChRs) located in the tissues of nervous, immune and other systems are a hot topic and much progress has been brought by X‐ray crystallography and cryo‐electron microscopy of these receptors. However, snake venom neurotoxins (“three‐finger proteins”, TFPs), which played a crucial role about 50 years ago in the isolation and characterization of the first nAChR, still are used as invaluable pharmacological tools for distinguishing different nAChR subtypes and for opening new ways in drug design. A much more novel trend is the TFPs of the Ly6 family which are endogenous regulators of the versatile functions of nAChRs.