Mapping the BKCa Channel's “Ca2+ Bowl”: Side-chains Essential for Ca

• Published in: The Journal of general physiology 2004-05, Vol.123 (5), p.475-489
• Main Authors: Bao, Lin, Kaldany, Christina, Holmstrand, Ericka C., Cox, Daniel H.
• Format: Article
• Language: English
• ISSN: 0022-1295; 1540-7748
• DOI: 10.1085/jgp.200409052

There is controversy over whether Ca 2+ binds to the BK Ca channel's intracellular domain or its integral-membrane domain and over whether or not mutations that reduce the channel's Ca 2+ sensitivity act at the point of Ca 2+ coordination. One region in the intracellular domain that has been implicated in Ca 2+ sensing is the “Ca 2+ bowl”. This region contains many acidic residues, and large Ca 2+ -bowl mutations eliminate Ca 2+ sensing through what appears to be one type of high-affinity Ca 2+ -binding site. Here, through site-directed mutagenesis we have mapped the residues in the Ca 2+ bowl that are most important for Ca 2+ sensing. We find acidic residues, D898 and D900, to be essential, and we find them essential as well for Ca 2+ binding to a fusion protein that contains a portion of the BK Ca channel's intracellular domain. Thus, much of our data supports the conclusion that Ca 2+ binds to the BK Ca channel's intracellular domain, and they define the Ca 2+ bowl's essential Ca 2+ -sensing motif. Overall, however, we have found that the relationship between mutations that disrupt Ca 2+ sensing and those that disrupt Ca 2+ binding is not as strong as we had expected, a result that raises the possibility that, when examined by gel-overlay, the Ca 2+ bowl may be in a nonnative conformation.