Disruption of fast inactivation did not abolish β1 modulation of slow inactivation. Fast inactivation was severely disrupted by the α-subunit F1311C mutant when channels were exposed to 500 μM internal MTS-ET. (A) Raw traces demonstrate the profound disruption of fast inactivation in the F1311C mutant exposed to internal MTS-ET. (B) In the absence of fast inactivation, coexpression of β1 (F1311C + β1, ■) caused an +11 mV depolarization (right-shift) in half-inactivation (V1/2) of slow inactivation compared to expression of mutant α-subunit alone/F1311C, ○), p < 0.005. Steady-state slow inactivation was measured as described in Fig 3, except that a 15 s conditioning pulse was used.