Versatile Regulation of Channel Inactivation

Abstract

Calcium enters cells through several classes of voltage-gated calcium channels to trigger numerous physiological responses. L-type and P/Q-type calcium channels undergo calcium-dependent inactivation (CDI), which depends on the calcium-dependent activation of calmodulin that is preassociated with the channel. In L-type channels, CDI is triggered by high-affinity calcium binding to the C lobe of calmodulin, whereas in P/Q-type channels CDI involves low-affinity calcium binding to the N lobe. Although N-type and R-type calcium channels contain carboxyl-terminal motifs required for calmodulin interactions and CDI ("IQ" and "EF-hand" motifs), these channels are generally believed to undergo solely voltage-dependent inactivation (see Zamponi). Liang et al . used spectrophotometric analysis to show that peptides spanning the IQ regions of N- and R-type channels bound dansyl-calmodulin; moreover, live-cell fluorescence resonance energy transfer (FRET) assays demonstrated that calmodulin bound to the appropriate segments of these two channels. Although R- and N-type channels did not show CDI (assessed by comparing calcium and barium currents) under standard conditions used in patch-clamp analysis (5 mM calcium chelator EGTA in the patch pipette), CDI was apparent when calcium buffering was reduced toward a more physiological level (0.5 mM EGTA). Mutational analysis combined with overexpression studies indicated that calmodulin was preassociated with the channels and served as the calcium sensor, acting through the N lobe. It is intriguing that calmodulin binding inhibited voltage-dependent inactivation. The authors proposed a model in which P/Q-, N-, and R-type channels undergo CDI in response to large, global increases in calcium, whereas L-type channels are sensitive to smaller localized changes produced by their own calcium influx. H. Liang, C. D. DeMaria, M. E. Erickson, M. X. Mori, B. A. Alseikhan, D. T. Yue, Unified mechanisms of Ca 2+ regulation across the Ca 2+ channel family. Neuron 39 , 951-960 (2003). [Online Journal] G. W. Zamponi, Calmodulin lobotomized: Novel insights into calcium regulation of voltage-gated channels. Neuron 39 , 879-881 (2003). [Online Journal]