Elimination of the BK(Ca) channel's high-affinity Ca(2+) sensitivity

Area of Science:

• Molecular Biology
• Biophysics
• Ion Channel Physiology

Background:

• The large-conductance calcium-activated potassium channel (BK(Ca)) plays a crucial role in regulating cellular excitability.
• Understanding the precise mechanisms of calcium (Ca2+) activation in BK(Ca) channels is essential for interpreting its physiological functions.

Purpose of the Study:

• To elucidate the specific calcium binding sites and their contribution to the activation of BK(Ca) channels.
• To characterize the affinity and properties of different calcium binding sites within the BK(Ca) channel.

Main Methods:

• Site-directed mutagenesis was employed to alter specific residues within the BK(Ca) channel.
• Electrophysiological recordings were used to assess the functional consequences of these mutations on channel activity.
• Energetic analyses were performed to quantify the binding characteristics of calcium ions.

Main Results:

• Mutations at the "Ca(2+) bowl" and residue M513 collectively eliminated the high-affinity Ca(2+) response of BK(Ca) channels.
• A low-affinity, magnesium (Mg2+)-sensitive Ca(2+) response remained after mutagenesis.
• Energetic analyses suggest the presence of three distinct Ca(2+) binding sites: one low-affinity (Mg2+-sensitive) and two high-affinity sites with similar binding properties.

Conclusions:

• The BK(Ca) channel possesses multiple Ca(2+) binding sites that contribute to its activation.
• Two high-affinity Ca(2+) binding sites, contributing equally, are critical for Ca(2+) to effectively gate the channel.
• This study provides quantitative estimates for the binding characteristics of these high-affinity sites.