DETERMINANTS OF APAMIN AND D-TUBOCURARINE BLOCK IN SK POTASSIUM CHANNELS

Small conductance calcium-activated potassium channels show a distinct pharmacology. Some, but not all, are blocked by the peptide toxin apa min, and apamin-sensitive channels are also blocked by d-tubocurarine. Cloned SK channels (small conductance calcium-activated potassium cha nnel) recapitulate these properties, We have investigated the structur al basis for these differences and found that two amino acid residues on either side of the deep pore are the primary determinants of sensit ivity to apamin and differential block by d-tubocurarine. Therefore, t he pharmacology of SK channels compared with other potassium channels correlates with structural differences in the outer pore region, Howev er, introduction of a tyrosine residue in the position analogous to th at which determines sensitivity to external tetraethylammonium for vol tage-gated potassium channels endows SK channels with an equivalent te traethylammonium sensitivity, indicating that the outer vestibules of the pores are similar, The pharmacology of channels formed in oocytes coinjected with SK1 and SK2 mRNAs, or with SK1-SK2 dimer mRNA, show th at SK subunits may form heteromeric channels.