Snake venom serine proteases (SVSPs) are complex and multifunctional enzymes, acting primarily on hemostasis. In this work, we report the hitherto unknown inhibitory effect of a SVSP, named collinein-1, isolated from the venom of Crotalus durissus collilineatus, on a cancer-relevant voltage-gated potassium channel (hEAG1). Among 12 voltage-gated ion channels tested, collinein-1 selectively inhibited hEAG1 currents, with a mechanism independent of its enzymatic activity. Corroboratively, we demonstrated that collinein-1 reduced the viability of human breast cancer cell line MCF7 (high expression of hEAG1), but does not affect the liver carcinoma and the non-tumorigenic epithelial breast cell lines (HepG2 and MCF10A, respectively), which present low expression of hEAG1. In order to obtain both functional and structural validation of this unexpected discovery, where an unusually large ligand acts as an inhibitor of an ion channel, a recombinant and catalytically inactive mutant of collinein-1 (His43Arg) was produced and found to preserve its capability to inhibit hEAG1. A molecular docking model was proposed in which Arg79 of the SVSP 99-loop interacts directly with the potassium selectivity filter of the hEAG1 channel.Voltage-gated potassium channels (Kv) are involved in a diversity of physiological processes, such as smooth muscle contraction 1 , cell volume control 2 , cell cycle progression 3 , cardiac repolarization 4 , and proliferation of tumor cells 5 . Several animal venom-related toxins are known to modulate Kv channel activity either by blocking 6 the ion selective pore or by modulating 7 the channel gating (i.e. opening and closing mechanisms). Pore blocking toxins bind to the external opening of the pore or to the internal cavity underneath the channel selectivity filter 8 , while gating modifiers induce conformational changes in the voltage-sensing domain of the channel, affecting the kinetics of channel opening and closing 9,10 . Most of the Kv-ligand toxins known to date are found in scorpion, spider, cone snail and sea anemone venoms 10-13 ; only relatively few snake venom toxins are known to interfere with ion channel activity, such as dendrotoxins, the B chain of β-bungarotoxin, and crotamine, acting on Kv channels [14][15][16][17][18] . Snake venom serine proteases (SVSPs) comprise a group of extensively studied toxins, widely found in the venom of terrestrial snakes from Viperidae, Elapidae, and Crotalidae families. Snake venom thrombin-like enzymes (SVTLEs) are the prevalent class of serine proteases from Viperidae venoms and present similar activity to that of human thrombin [19][20][21] . SVSPs may be considered multifunctional toxins due to their broad substrate specificity and can thus act on different systems of the prey or victim organisms 22 . Therefore, the investigation of the intrinsic pathways involved in the variety of biological activities of these molecules may contribute to open Scientific RepoRtS | (2020) 10:4476 | https://doi.org/10.1038/s41598-020-61258-x www.nature.com/scient...