Regulation of single NMDA receptor channel activity by alpha-actinin and calmodulin in rat hippocampal granule cells

The NMDA receptor is modulated by changes in the intracellular calcium concentration, through activation of various intracellular calcium-dependent proteins. We have investigated regulation of single NMDA receptor channel activity by the calcium-sensing proteins alpha-actinin and calmodulin. Both of these proteins bind to the NMDA receptor NR1 subunit C-terminus at the CO region where they compete for occupation of the CO site and contribute to calcium-dependent inactivation of NMDA receptor-mediated whole-cell currents. Calmodulin has also been shown to bind to the neighbouring C I region where it has been shown to reduce single channel open time. To investigate regulation of single NMDA channel activity by alpha-actinin and calmodulin, we selected concentrations of these two proteins that would result in maximal binding to the CO region and/or the C1 region in the case of calmodulin. Alpha-actinin binding was found to predominantly decrease single channel shut time, resulting in an increased open probability (P-open), whereas calmodulin binding reduced single channel mean open time, resulting in an overall reduction in P-open. The physiological implications of these findings are discussed.