Calcium channel γ6 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, γ4, γ6 and γ7, whose function(s) in the heart are unknown. Here we compare the effects of the previously uncharacterized γ6 subunit with that of γ4 and γ7 on a low voltage-activated calcium channel (Cav3.1) that is expressed in cardiac myoctyes. Co-expression of both the long and short γ6 subunit isoforms, γ6L and γ6S, with Cav3.1 in HEK-293 cells significantly decreases current density by 49% and 69%, respectively. Two other γ subunits expressed in cardiac myocytes, γ4 and γ7, have no significant effect on Cav3.1 current. Neither γ6L, γ6S, γ4 nor γ7 significantly affect the voltage dependency of activation or inactivation or the kinetics of Cav3.1 current. Transient expression of γ6L 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 γ6L is localized primarily in HEK-293 cell surface membranes where it is evenly distributed. Expression of γ6L 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 γ6 subunit has a unique ability to inhibit Cav3.1 dependent calcium current that is not shared with the γ4 and γ7 isoforms and is thus a potential regulator of cardiac low voltage-activated calcium current.