Chillin’ Channels Through Internalization

Abstract

Voltage-gated calcium channels, and L-type calcium channels (LTCs) in particular, provide a key mechanism by which electrical depolarization of a neuron is translated into changes in biochemical signaling systems that alter cellular function and gene expression. Green et al. show that, beyond the acute Ca 2+ -dependent inhibition of LTC conductance by calmodulin, there is an additional mechanism providing a longer-term negative feedback--Ca 2+ -dependent internalization of the channel. The authors modified the LTC Ca v 1.2 with tags in an extracellular loop and on the intracellular N terminus, which allowed them to monitor the fraction of the channel protein that was present on the surface of individual cultured cortical neurons. Electrical depolarization of the neurons decreased the fraction of the channels that was present on the cell surface. Total internal reflection microscopy showed that the channels became internalized in endosomal vesicles. The authors identified a fragment of the channel that produced a dominant-negative inhibition of internalization and, assuming it might be sequestering a protein that regulated the channel, used it in a two-hybrid screen to find an unexpected interacting protein, eIF3e. The eIF3e protein gets its name from its association with the elongation initiation complex, but it also interacts with the proteasome and the COP9 signalosome, complexes that function to degrade or regulate proteins modified by ubiquitination. At least when overexpressed, eIF3e interacted with Ca v 1.2 in cells, and increased amounts of eIF3e were associated with immunoprecipitated Ca v 1.2 when cells had been depolarized. In commentary on the paper, Varela and Zamponi note that a C-terminal fragment of Ca v 1.2 is reported to move to the nucleus and function as a transcription factor. Might eIF3e help route the channel to a compartment where proteolytic release of the C terminus would be activated for control of gene expression? Even if this intriguing scenario does not pan out, it seems likely that alterations in the density of LTCs at the cell surface is a mechanism that will affect a range of physiological processes, from cardiac function to neuronal plasticity. E. M. Green, C. F. Barrett, G. Bultynck, S. M. Shamah, R. E. Dolmetsch, The tumor suppressor eIF3e mediates calcium-dependent internalization of the L-type calcium channel Ca v 1.2. Neuron 55 , 615-632 (2007). [Online Journal] D. Varela, G. W. Zamponi, Use ‘em and lose ‘em--Activity-induced removal of calcium channels from the plasma membrane. Neuron 55 , 539-541 (2007). [Online Journal]