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

Abstract

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.