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-e lectrode 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 po tential axis. The results were interpreted in terms of the surface cha rge 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 res idues on the extracellular loop between transmembrane segment 5 and th e pore region constitutes the functional surface charges. The results further suggest that the surface charge density plays an important rol e in controlling the activation voltage range.