Calmodulin Binding to the C-Terminus of the Small-Conductance Ca2+-Activated K+ Channel hSK1 Is Affected by Alternative Splicing

We identified three splice variants of hSK1 whose C-terminal structures are determined by the independent deletion of two contiguous nucleotide sequences. The upstream sequence extends 25 bases in length, is initiated by a donor splice site within exon 8, and terminates at the end of the exon. The downstream sequence consists of nine bases that compose exon 9. When the upstream sequence (hSK1-25b) or both sequences (hSK1-34b) are deleted, truncated proteins are encoded in which the terminal 118 amino acids are absent. The binding of calmodulin to these variants is diminished, particularly in the absence of Ca2+ ions. The first 20 amino acids of the segment deleted from hSK1-25b and hSK1-34b contain a 1−8−14 Ca2+-calmodulin binding motif, and synthetic oligopeptides based on this region bind calmodulin better in the presence than absence of Ca2+ ions. When the downstream sequence (hSK1-9b) alone is deleted, only the three amino acids A452, Q453, and K454 are removed, and calmodulin binding is not reduced. On the basis of the relative abundance of mRNA encoding each of the four isoforms, the full-length variant appears to account for most hSK1 in the human hippocampus, while hSK1-34b predominates in reticulocytes, and hSK1-9b is especially abundant in human erythroleukemia cells in culture. We conclude that the binding of calmodulin by hSK1 can be modulated through alternative splicing.