Role of Repeat I in the fast inactivation kinetics of the Ca(V)2.3 channel

The molecular basis for inactivation in Ca(V)2.3 (alpha 1E) channels was studied after expression of alpha 1E/alpha 1C (Ca(V)2.3/Ca(V)1.2) chimeras in Xenopus oocytes. In the presence of 10 mM Ba(2+), the CEEE chimera (Repeat I+part of the I-II linker from Ca(V)1.2) displayed inactivation properties similar to Ca(V)1.2 despite being more than 90% homologous to Ca(V)2.3. The transmembrane segments of Repeat I did not appear to be crucial as inactivation of EC(IS1-6)EEE was not significantly different than Ca(V)2.3. In contrast, EC(AID)EEE, with the beta-subunit binding domain from Ca(V)1.2, tended to behave like Ca(V)1.2 in terms of inactivation kinetics and voltage dependence. A detailed kinetic analysis revealed nonetheless that CEEE and EC(AID)EEE retained the fast inactivation time constant (tau(fast) approximately equal to 20-30 ms) that is a distinctive feature of Ca(V)2.3. Altogether, these data suggest that the region surrounding the AID binding site plays a pivotal albeit not exclusive role in determining the inactivation properties of Ca(V)2.3.