Weak neurotoxins from snake venom are small proteins with five disulfide bonds, which have been shown to be poor binders of nicotinic acetylcholine receptors. We report on the cloning and sequencing of four cDNAs encoding weak neurotoxins from Naja sputatrix venom glands. The protein encoded by one of them, Wntx-5, has been synthesized by solid-phase synthesis and characterized. The physicochemical properties of the synthetic toxin (sWntx-5) agree with those anticipated for the natural toxin. We show that this toxin interacts with relatively low affinity (K(d) = 180 nm) with the muscular-type acetylcholine receptor of the electric organ of T. marmorata, and with an even weaker affinity (90 microm) with the neuronal alpha7 receptor of chicken. Electrophysiological recordings using isolated mouse hemidiaphragm and frog cutaneous pectoris nerve-muscle preparations revealed no blocking activity of sWntx-5 at microm concentrations. Our data confirm previous observations that natural weak neurotoxins from cobras have poor affinity for nicotinic acetylcholine receptors.