Modulation of CaV2.1 Channels by the Neuronal Calcium-Binding Protein Visinin-Like Protein-2

Abstract

Ca V 2.1 channels conduct P/Q-type Ca 2+currents that are modulated by calmodulin (CaM) and the structurally related Ca 2+-binding protein 1 (CaBP1). Visinin-like protein-2 (VILIP-2) is a CaM-related Ca 2+-binding protein expressed in the neocortex and hippocampus. Coexpression of Ca V 2.1 and VILIP-2 in tsA-201 cells resulted in Ca 2+channel modulation distinct from CaM and CaBP1. Ca V 2.1 channels with β 2a subunits undergo Ca 2+-dependent facilitation and inactivation attributable to association of endogenous Ca 2+/CaM. VILIP-2 coexpression does not alter facilitation measured in paired-pulse experiments but slows the rate of inactivation to that seen without Ca 2+/CaM binding and reduces inactivation of Ca 2+currents during trains of repetitive depolarizations. Ca V 2.1 channels with β 1b subunits have rapid voltage-dependent inactivation, and VILIP-2 has no effect on the rate of inactivation or facilitation of the Ca 2+current. In contrast, when Ba 2+replaces Ca 2+as the charge carrier, VILIP-2 slows inactivation. The effects of VILIP-2 are prevented by deletion of the CaM-binding domain (CBD) in the C terminus of Ca V 2.1 channels. However, both the CBD and an upstream IQ-like domain must be deleted to prevent VILIP-2 binding. Our results indicate that VILIP-2 binds to the CBD and IQ-like domains of Ca V 2.1 channels like CaM but slows inactivation, which enhances facilitation of Ca V 2.1 channels during extended trains of stimuli. Comparison of VILIP-2 effects with those of CaBP1 indicates striking differences in modulation of both facilitation and inactivation. Differential regulation of Ca V 2.1 channels by CaM, VILIP-2, CaBP1, and other neurospecific Ca 2+-binding proteins is a potentially important determinant of Ca 2+entry in neurotransmission.