HEK293 were cultured in DMEM medium with 10% fetal bovine serum and, for transfection, confluent HEK293 cells were plated on 60 mm Petri dishes and transfected using PolyFect transfection reagent (Qiagen,Balencia, CA) complexed with BK variants (BK-Insertless, BK-ALCOREX, BK-STREX) in pVAX vector (Invitrogen) cDNAs.