Multiple structural domains contribute to voltage-dependent inactivation of rat brain alpha(1E) calcium channels

We have investigated the molecular determinants that mediate the difference s in voltage-dependent inactivation properties between rapidly inactivating (R-type) alpha(1E) and noninactivating (L-type) alpha(1C) calcium channels . When coexpressed in human embryonic kidney cells with ancillary beta(1b) and alpha(2)-delta subunits, the wild type channels exhibit dramatically di fferent inactivation properties; the half-inactivation potential of alpha(1 E) is 45 mV more negative than that observed with alpha(1C), and during a 1 50-ms test depolarization, alpha(1E) undergoes 65% inactivation compared wi th only about 15% for alpha(1C). To define the structural determinants that govern these intrinsic differences, we have created a series of chimeric c alcium channel alpha(1) subunits that combine the major structural domains of the two wild type channels, and we investigated their voltage-dependent inactivation properties. Each of the four transmembrane domains significant ly affected the half-inactivation potential, with domains II and III being most critical. In particular, substitution of alpha(1C) sequence in domains II or III with that of alpha(1E) resulted in 25-mV negative shifts in half -inactivation potential. Similarly, the differences in inactivation rate we re predominantly governed by transmembrane domains II and III and to some e xtent by domain TV. Thus, voltage-dependent inactivation of alpha(1E) chann els is a complex process that involves multiple structural domains and poss ibly a global conformational change in the channel protein.