Ca(2+)-sensitive inactivation of L-type Ca(2+ )channels depends on multiple cytoplasmic amino acid sequences of the α(1C) subunit

Ca(2+)-dependent inactivation of Ca(2+) currents is a physiological phenomenon widely associated with L-type Ca(2+) channels. Although the pore-forming α(1C) subunit of the channel is the target for Ca(2+) binding, the amino acid sequences involved in the binding and/or in the coordination of Ca(2+)-dependent inactivation are still unclear. Based on previous experiments, we have prepared truncation mutants of a human α(1C )subunit by systematically deleting an EF-hand motif and sequences in a segment of 80 amino acids in the carboxyl-terminal tail. We found that the rate as well as the Ca(2+) dependence of inactivation of currents through these mutated channels were very different. We have identified three amino acid sequences, the presence of which is important for Ca(2+)-dependent inactivation: (i) a putative Ca(2+)-binding EF-hand motif, (ii) two hydrophilic residues (asparagine and glutamic acid) 77–78 amino acids downstream of the EF-hand motif, and (iii) a putative IQ calmodulin binding motif. We suggest that Ca(2+)-dependent inactivation is a cooperative process involving several amino acid sequences in cytoplasmic segments of the α(1C) subunit.