Divalent Cation Effects on the Shaker K Channel Suggest a Pentapeptide Sequence as Determinant of Functional Surface Charge Density

The effects of the divalent cations strontium and magnesium on Shaker K channels expressed in Xenopus oocytes were investigated with a two-electrode voltage-clamp technique. 20 mm of the divalent cation shifted activation (conductance vs. potential), steady-state inactivation and inactivation time constant vs. potential curves 10–11 mV along the potential axis. The results were interpreted in terms of the surface charge theory, and the surface charge density was estimated to be −0.27 e nm⁻². A comparison of primary structure data and experimental data from the present and previous studies suggests that the first five residues on the extracellular loop between transmembrane segment 5 and the pore region constitutes the functional surface charges. The results further suggest that the surface charge density plays an important role in controlling the activation voltage range. Received: 12 November 1997/Revised: 1 June 1998