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Slo1 Tail Domains, but Not the Ca2+ Bowl, Are Required for the ß1 Subunit to Increase the Apparent Ca2+ Sensitivity of BK Channels
Address correspondence to Dr. Karl L. Magleby, Department of Physiology and Biophysics, University of Miami School of Medicine, P.O. Box 016340, Miami, FL 33101-6430. E-mail: kmagleby{at}miami.edu
Functional large-conductance Ca2+- and voltage-activated K+ (BK) channels can be assembled from four
subunits (Slo1) alone, or together with four auxiliary ß1 subunits to greatly increase the apparent Ca2+ sensitivity of the channel. We examined the structural features involved in this modulation with two types of experiments. In the first, the tail domain of the subunit, which includes the RCK2 (regulator of K+ conductance) domain and Ca2+ bowl, was replaced with the tail domain of Slo3, a BK-related channel that lacks both a Ca2+ bowl and high affinity Ca2+ sensitivity. In the second, the Ca2+ bowl was disrupted by mutations that greatly reduce the apparent Ca2+ sensitivity. We found that the ß1 subunit increased the apparent Ca2+ sensitivity of Slo1 channels, independently of whether the subunits were expressed as separate cores (S0-S8) and tails (S9-S10) or full length, and this increase was still observed after the Ca2+ bowl was mutated. In contrast, ß1 subunits no longer increased Ca2+ sensitivity when Slo1 tails were replaced by Slo3 tails. The ß1 subunits were still functionally coupled to channels with Slo3 tails, as DHS-I and 17 ß-estradiol activated these channels in the presence of ß1 subunits, but not in their absence. These findings indicate that the increase in apparent Ca2+ sensitivity induced by the ß1 subunit does not require either the Ca2+ bowl or the linker between the RCK1 and RCK2 domains, and that Slo3 tails cannot substitute for Slo1 tails. The ß1 subunit also induced a decrease in voltage sensitivity that occurred with either Slo1 or Slo3 tails. In contrast, the ß1 subunit–induced increase in apparent Ca2+ sensitivity required Slo1 tails. This suggests that the allosteric activation pathways for these two types of actions of the ß1 subunit may be different.
Key Words: Ca2+-activated K+ channel • RCK domain • DHS-I • estrogen • Slo3
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