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An evolutionarily conserved binding site for serine proteinase inhibitors in large conductance calcium-activated potassium channels.

Complementary DNA coding for the channel-forming alpha-subunit of a large conductance Ca(2+)-activated K+ channel (maxi Kca channel) was cloned from bovine aortic smooth muscle cells. This cloned mammalian KCa channel (Bslo) and its homolog from Drosophila (Dslo) were expressed in the HEK293 human embryonic kidney cell line. Both Bslo and Dslo KCa channels were sensitive to inhibition by the internally applied serine proteinase inhibitors: bovine pancreatic trypsin inhibitor (BPTI, KD = 7.0 microM for Bslo and 2.6 microM for Dslo) and chicken ovoinhibitor (OI, KD = 1.5 microM for Bslo and 11.4 microM for Dslo). BPTI and OI are members of the Kunitz and Kazal families of proteinase inhibitors, respectively. The approximately 60-residue inhibitory domains of these proteins have a different tertiary structure except in the region of a loop formed by approximately 6 residues, in which the peptide backbone adopts a similar conformation complementary to the active site cleft of many serine proteinases. At the single-channel level, BPTI and OI were found to inhibit KCa channels by a similar mechanism involving the production of discrete low-conductance events. These two inhibitors also exhibited competitive behavior, suggesting that they bind to an overlapping site. Kinetic characterization revealed that the dissociation rate of BPTI from the bovine KCa channel is fast (k(off) = 0.41 s-1), whereas that from the Drosophila KCa channel is slow (k(off) = 9.0 x 10(-4) s-1) and indicative of a strong molecular interaction. The stable complex of BPTI and trypsin was inactive as a KCa channel inhibitor, further supporting the idea that the trypsin inhibitory loop of BPTI recognizes a specific site on the channel protein. These results lead to the conclusion that the alpha-subunit of maxi KCa channels contains a conserved proteinase inhibitor binding site. We hypothesize that this site corresponds to a C-terminal domain of the channel protein that structurally resembles serine proteinases.