Lanthanide gadolinium (Gd3+) blocks CaV1.2 channels at the selectivity filter. Here we investigated whether Gd3+ block interferes with Ca2+-dependent inactivation, which requires Ca 2+ entry through the same site. Using brief pulses to 200 mV that relieve Gd3+ block but not inactivation, we monitored how the proportions of open and open-blocked channels change during inactivation. We found that blocked channels inactivate much less. This is expected for Gd 3+ block of the Ca2+ influx that enhances inactivation. However, we also found that the extent of Gd3+ block did not change when inactivation was reduced by abolition of Ca2+/calmodulin interaction, showing that Gd3+ does not block the inactivated channel. Thus, Gd3+ block and inactivation are mutually exclusive, suggesting action at a common site. These observations suggest that inactivation causes a change at the selectivity filter that either hides the Gd3+ site or reduces its affinity, or that Ca2+ occupies the binding site at the selectivity filter in inactivated channels. The latter possibility is supported by previous findings that the EEQE mutation of the selectivity EEEE locus is void of Ca2+-dependent inactivation (Zong Z.Q., J.Y. Zhou, and T. Tanabe. 1994. Biochem. Biophys. Res. Commun. 201:1117-11123), and that Ca2+-inactivated channels conduct Na+ when Ca2+ is removed from the extracellular medium (Babich O., D. Isaev, and R. Shirokov. 2005. J. Physiol. 565:709-717). Based on these results, we propose that inactivation increases affinity of the selectivity filter for Ca2+ so that Ca2+ ion blocks the pore. A minimal model, in which the inactivation "gate" is an increase in affinity of the selectivity filter for permeating ions, successfully simulates the characteristic U-shaped voltage dependence of inactivation in Ca2+.
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