The measured osmolarity of the external solution was between 302 and 308 mOsm. The internal solution consisted

of (in mM) 140 KF, 2 MgCl2, 1 CaCl2, 10 HEPES, and 11 EGTA, pH 7.22. Measurements were performed using Axopatch 200A amplifiers connected to Axon Digidata 1200 data acquisition hardware (Molecular Devices, Daporinad Sunnyvale, CA). Pipettes were pulled from GC 150 F-15 borosilicate glass resulting in electrodes having 3–5 MΩ resistance in the bath. For data acquisition and analysis, the pClamp9/10 software package (Molecular Devices) was used. Before analysis, current traces were corrected for ohmic leak and digitally filtered (three-point boxcar smoothing). Each data point on dose-response curve represents the mean of 3 independent experiments, and error bars represent standard error of the mean. Data points on the dose-response curve were fitted with a two parameter Hill-equation: RF = KdH/(KdH + [Tx]H), where RF is the Remaining Current Fraction (calculated as I/I0, where I is the peak current measured in the presence of toxin and I0 is the peak current in control solution), Kd is the dissociation constant, H is the Hill-coefficient and [Tx] is the toxin concentration. Kd was also determined from

Lineweaver–Burk analysis (1/RF vs 1/[Tx]). Fig. 1A shows the RP-HPLC chromatographic profile of O. cayaporum venom separated in an analytical column. Sixty different chromatographic fractions were obtained. The fraction eluting at 21.22 min was further purified in an analytical C18 reversed phase column given a major component, labeled with an asterisk GPCR Compound Library in the Fig. 1B.This component under mass spectrometry analysis showed Verteporfin the presence of a single component with molecular mass of 3807 atomic mass units (a.m.u.) ( Fig. 1C). The automatic amino acid sequence of the peptide gave a unique sequence, as indicated

in Fig. 2. The theoretical molecular mass obtained for this amino acid sequence was 3806.61, very close to the experimentally obtained value. OcyKTx2 is a basic peptide with an isoelectric point (pI) of 8.92. On the basis of chain length, number of disulfide bridges, sequence similarity and the conditions established by [29], OcyKTx2 belongs to the subfamily α-KTx6, containing four disulfide-bridges (Fig. 2), and we propose its systematic classification as α-KTx6.17. The phylogenetic analysis built by the Maximum Parsimony (MP) method is presented in Fig. 3 that shows the results of an unrooted phylogenetic tree, where it was possible to group the OcyKTx2 into the same branch of most of α-KTx6 peptides, supporting its classification as α-KTx6.17. The physiological effect of OcyKTx2 was investigated in the Sf9 cell culture system, expressing the Shaker B K+-channel, and in the human lymphocyte expressing Kv1.3 channel, as shown in Fig. 4. The traces in Fig. 4A show that the addition of 1 μM OcyKTx2 to the bath solution completely and reversibly inhibits the K+ current through Shaker-B channels.