Muscarinic toxin‐like proteins from cobra venom

Abstract

Three new polypeptides were isolated from the venom of the Thailand cobra Naja kaouthia and their amino‐acid sequences determined. They consist of 65‐amino‐acid residues and have four disulfide bridges. A comparison of the amino‐acid sequences of the new polypeptides with those of snake toxins shows that two of them (MTLP‐1 and MTLP‐2) share a high degree of similarity (55–74% sequence identity) with muscarinic toxins from the mamba. The third polypeptide (MTLP‐3) is similar to muscarinic toxins with respect to the position of cysteine residues and the size of the disulfide‐confined loops, but shows less similarity to these toxins (30–34% sequence identity). It is almost identical with a neurotoxin‐like protein from Bungarus multicinctus (TrEMBL accession number Q9W727), the sequence of which has been deduced from cloned cDNA only. The binding affinities of the isolated muscarinic toxin‐like proteins towards the different muscarinic acetylcholine receptor (mAChR) subtypes (m1–m5) was determined in competition experiments with N ‐[ 3 H]methylscopolamine using membrane preparations from CHO‐K1 cells, which express these receptors. We found that MTLP‐1 competed weakly with radioactive ligand for binding to all mAChR subtypes. The most pronounced effect was observed for the m3 subtype; here an IC 50 value of about 3 µ m was determined. MTLP‐2 had no effect on ligand binding to any of the mAChR subtypes at concentrations up to 1 µ m . MTLP‐1 showed no inhibitory effect on α‐cobratoxin binding to the nicotinic acetylcholine receptor from Torpedo californica at concentrations up to 20 µ m .