Blocks transient outward voltage-gated potassium channels in rat ventricular myocytes (thus prolonging action-potential duration) and rat Kv4.2/KCNA4 channels expressed in Xenopus oocytes. Is also a weak blocker of calcium channels in rat cerebellar granule cells. Expressed by the venom gland. The presence of a 'disulfide through disulfide knot' structurally defines this protein as a knottin. Belongs to the neurotoxin 10 (Hwtx-1) family. 21 (HpTx1) subfamily.