Calcium channel γ ₆ subunits are unique modulators of low voltage-activated (Cav3.1) calcium current

The calcium channel gamma (γ) subunit family consists of eight members whose functions include modulation of high voltage-activated (HVA) calcium currents in skeletal muscle and neurons, and regulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propanoic acid (AMPA) receptor targeting. Cardiac myocytes express at least three γ subunits, γ ₄, γ ₆ and γ ₇, whose function(s) in the heart are unknown. Here we compare the effects of the previously uncharacterized γ ₆ subunit with that of γ ₄ and γ ₇ on a low voltage-activated calcium channel (Cav3.1) that is expressed in cardiac myoctyes. Co-expression of both the long and short γ ₆ subunit isoforms, γ _6L and γ _6S, with Cav3.1 in HEK-293 cells significantly decreases current density by 49% and 69%, respectively. Two other γ subunits expressed in cardiac myocytes, γ ₄ and γ ₇, have no significant effect on Cav3.1 current. Neither γ _6L, γ _6S, γ ₄ nor γ ₇ significantly affect the voltage dependency of activation or inactivation or the kinetics of Cav3.1 current. Transient expression of γ _6L in an immortalized atrial cell line (HL-1) significantly reduces the endogenous low voltage-activated current in these cells by 63%. Green fluorescent protein tagged γ _6L is localized primarily in HEK-293 cell surface membranes where it is evenly distributed. Expression of γ _6L does not affect the level of Cav3.1 mRNA or the amount of total Cav3.1 protein in transfected HEK-293 cells. These results demonstrate that the γ ₆ subunit has a unique ability to inhibit Cav3.1 dependent calcium current that is not shared with the γ ₄ and γ ₇ isoforms and is thus a potential regulator of cardiac low voltage-activated calcium current.