Were amplified (Verities PCR, Applied Biosystems, Austin, TX, USA), the PCR products were purified (Exosap-IT, USB, Isogen Life Science, The Netherlands) and they were directly sequenced in both directions (Big Dye Terminator v3.1 cycle sequencing Kit and 3130XL Genetic Analyzer, both from Applied Biosystems). The DNA sequence was compared with the reference sequence NM000335 for SCN5A (OMIM601144) (UCSC Genome binformatics [12]/NCBI-Mendelian Inheritance [13]). DNA samples from 300 healthy Spanish individuals (600 alleles) were used as control samples.Site-directed MutagenesisThe wild-type (WT) human SCN5A cDNA (Uniprot reference: Q14524) cloned in pcDNA3.1 (a kind gift from Dr. Matteo Vatta, Baylor College of Medicine, Houston, TX, USA.) was used as template to engineer the mutation I890T using the QuikChange Site-Directed Mutagenesis system (Stratagene, La Jolla, CA, USA) and the following primers (mutation underlined): 59-GCCTTCCTCACCATCTTCCGCATCCTCTGTGGAGAGTGGATCG-39 and. 59 CGGAAGATGGTGAGGAAGGCATGAAAGAAGTCCATCATGTGC-39. The resultant construct was directly sequenced to verify the presence of the desired mutation and the absence of additional variations. pcDNA3.1 included a FLAG tag (sequence: 520-26-3 DYKDDDDK between prolines P154 and P155 of SCN5A) which has been previously shown not to alter the Nav1.5 current properties [14,15]).Cell Surface Protein BiotinylationCells were washed with Dulbecco’s Phosphate-Buffered Saline (DPBS) supplemented with 0.9 mM CaCl2 and 0.49 mM MgCl2 (DPBS+) at pH 7.4. Membrane proteins were biotinylated by incubating cells with 1.6?.5 mg/ml of EZ-link sulfo-NHS-LCLC-biotin (Pierce, Thermo Scientific, Rockford, IL, USA) in DPBS+ for 30 min at 4uC. Cells were then washed 3 times in DPBS+ with 100 mM glycine, then with DPBS+ containing 20 mM glycine, and scrapped in Triton X-100 lysis buffer (1 Triton X-100, 50 mM Tris/HCl pH 7.4, 150 mM NaCl, 1 mM EDTA and Complete Protease Inhibitor Cocktail (Roche, Madrid, Spain)). Lysates were obtained after 1 h rotating at 4uC. Insoluble materials were removed by centrifugation. Supernatants were incubated with Ultralink Immobilised NeutrAvidin beads (Pierce) overnight at 4uC. The beads were precipitated and washed with Triton X-100 lysis buffer, then in saline solution (5 mM EDTA, 350 mM NaCl and 0.1 TX-100 in DPBS+ pH 7.4) and Benzocaine finally in 10 mM Tris/HCl pH 7.4. Precipitated beads were resuspended in SDS-PAGE loading buffer and heated for 
5 min at 70uC. Proteins were resolved in 4 SDS-PAGE gels and transferred to PVDF membranes (Millipore, Billerica, MA, USA). Membranes were probed with a rabbit anti-human Nav1.5 antibody (anti-hNav1.5; Alomone Labs, 24786787 Jerusalem, Israel) at a dilution of 1:1,000, overnight at 4uC. A secondary horseradish peroxidase-conjugated antibody (Thermo Scientific, Rockford, IL, USA) was used at a dilution of 1:2,000 for 1 h at room temperature, and signals wereCell Culture and TransfectionHuman embryonic kidney (HEK) 293 cells, a kind 
gift from Dr. Miguel Valverde [16], were used as experimental model. Cells were maintained in Dulbecco’s Modified Eagle’s Medium supplemented with 10 Fetal Bovine Serum, 1 antibioticantimycotic and 1 Glutamax (all from Invitrogen, Carlsbad, CA, USA) at 37uC and 5 CO2. HEK cells were transiently transfected with 2.9 mg of the SCN5A construct, either WT or I890T, using GeneCellinTM Transfection Reagent (BioCellChallenge, Toulon Cedex, France) following the manufacturer’s specifications. Co-transfection with 0.1 mg of.Were amplified (Verities PCR, Applied Biosystems, Austin, TX, USA), the PCR products were purified (Exosap-IT, USB, Isogen Life Science, The Netherlands) and they were directly sequenced in both directions (Big Dye Terminator v3.1 cycle sequencing Kit and 3130XL Genetic Analyzer, both from Applied Biosystems). The DNA sequence was compared with the reference sequence NM000335 for SCN5A (OMIM601144) (UCSC Genome binformatics [12]/NCBI-Mendelian Inheritance [13]). DNA samples from 300 healthy Spanish individuals (600 alleles) were used as control samples.Site-directed MutagenesisThe wild-type (WT) human SCN5A cDNA (Uniprot reference: Q14524) cloned in pcDNA3.1 (a kind gift from Dr. Matteo Vatta, Baylor College of Medicine, Houston, TX, USA.) was used as template to engineer the mutation I890T using the QuikChange Site-Directed Mutagenesis system (Stratagene, La Jolla, CA, USA) and the following primers (mutation underlined): 59-GCCTTCCTCACCATCTTCCGCATCCTCTGTGGAGAGTGGATCG-39 and. 59 CGGAAGATGGTGAGGAAGGCATGAAAGAAGTCCATCATGTGC-39. The resultant construct was directly sequenced to verify the presence of the desired mutation and the absence of additional variations. pcDNA3.1 included a FLAG tag (sequence: DYKDDDDK between prolines P154 and P155 of SCN5A) which has been previously shown not to alter the Nav1.5 current properties [14,15]).Cell Surface Protein BiotinylationCells were washed with Dulbecco’s Phosphate-Buffered Saline (DPBS) supplemented with 0.9 mM CaCl2 and 0.49 mM MgCl2 (DPBS+) at pH 7.4. Membrane proteins were biotinylated by incubating cells with 1.6?.5 mg/ml of EZ-link sulfo-NHS-LCLC-biotin (Pierce, Thermo Scientific, Rockford, IL, USA) in DPBS+ for 30 min at 4uC. Cells were then washed 3 times in DPBS+ with 100 mM glycine, then with DPBS+ containing 20 mM glycine, and scrapped in Triton X-100 lysis buffer (1 Triton X-100, 50 mM Tris/HCl pH 7.4, 150 mM NaCl, 1 mM EDTA and Complete Protease Inhibitor Cocktail (Roche, Madrid, Spain)). Lysates were obtained after 1 h rotating at 4uC. Insoluble materials were removed by centrifugation. Supernatants were incubated with Ultralink Immobilised NeutrAvidin beads (Pierce) overnight at 4uC. The beads were precipitated and washed with Triton X-100 lysis buffer, then in saline solution (5 mM EDTA, 350 mM NaCl and 0.1 TX-100 in DPBS+ pH 7.4) and finally in 10 mM Tris/HCl pH 7.4. Precipitated beads were resuspended in SDS-PAGE loading buffer and heated for 5 min at 70uC. Proteins were resolved in 4 SDS-PAGE gels and transferred to PVDF membranes (Millipore, Billerica, MA, USA). Membranes were probed with a rabbit anti-human Nav1.5 antibody (anti-hNav1.5; Alomone Labs, 24786787 Jerusalem, Israel) at a dilution of 1:1,000, overnight at 4uC. A secondary horseradish peroxidase-conjugated antibody (Thermo Scientific, Rockford, IL, USA) was used at a dilution of 1:2,000 for 1 h at room temperature, and signals wereCell Culture and TransfectionHuman embryonic kidney (HEK) 293 cells, a kind gift from Dr. Miguel Valverde [16], were used as experimental model. Cells were maintained in Dulbecco’s Modified Eagle’s Medium supplemented with 10 Fetal Bovine Serum, 1 antibioticantimycotic and 1 Glutamax (all from Invitrogen, Carlsbad, CA, USA) at 37uC and 5 CO2. HEK cells were transiently transfected with 2.9 mg of the SCN5A construct, either WT or I890T, using GeneCellinTM Transfection Reagent (BioCellChallenge, Toulon Cedex, France) following the manufacturer’s specifications. Co-transfection with 0.1 mg of.