BK Channel (IMAGE) Washington University in St. Louis Caption Four protein subunits assemble themselves into a rough cube with the pore in the middle and voltage sensors hanging off the corners. (Only two subunits are shown here.) Helices line the pore, interlocking at the bottom to close it. The tail ends of the protein, which contain the calcium binding sites (Ca2+), end up in a ring-like stucture beneath the pore called the cystolic domain. Credit Jianmin Cui Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.