In the yeast potassium channel, Tok1p, the external ring of aspartate residues modulates both gating and conductance.

The yeast plasma-membrane potassium channel, Tok1p, is a voltage-dependent outward rectifier, the gating and steady-state conductance of which are conspicuously modulated by extracellular [K(+)] (K(+)). Activation is slow at high K(+), showing time constants (tau(a)) of approximately 90 ms when K(+) is 150 mM (depolarizing step to +100 mV), and inactivation is weak (<30%) during sustained depolarization. Lowering K(+) accelerates activation, increases peak current, and enhances inactivation, so that at 15 mM K(+) tau(a) is less than 50 ms and inactivation suppresses approximately 60% of peak current. Two negative residues, Asp292 and Asp426, near the mouth of the assembled channel, modulate both kinetics and conductance of the channel. Charge neutralization in the mutant Asp292Asn allows fast activation (tau(a) approximately 20 ms) at high K(+), peak currents diminishing with decreasing K(+), and fast, nearly complete, inactivation. The voltage dependence of tau(a) persists in the mutant, but the K(+) dependence almost disappears. Similar but smaller changes are seen in the Asp426Asn mutant, implying that pore geometry in the functional channel has twofold, not fourfold, symmetry.