Effect of bestrophin-1 on L-type Ca2+ channel activity depends on the Ca2+ channel beta-subunit.

Best's vitelliforme macular degeneration is an inherited retinal degeneration associated with a reduction of the light-peak in the patient's electro-oculogram. Bestrophin-1, the product of the disease-promoting/forming gene can function as regulator of voltage-dependent L-type Ca(2+) channels in the retinal pigment epithelium (RPE). Since mice deficient for either β4-subunits or Ca(V)1.3 subunits show reduced light-peaks, the regulatory function of bestrophin-1 on heterologously expressed Ca(2+) channels composed of the pore-forming Ca(V)1.3 and the auxiliary β4-subunit was analyzed. Precipitation of β4-subunits led to co-precipitation with bestrophin-1 and subsequent analysis of subcellular localization showed co-localization of bestrophin-1, Ca(V)1.3 and β4-subunit in the cell membrane. Ca(V)1.3 currents in the presence of β4-subunits and bestrophin-1 showed accelerated time-dependent activation and decreased current density compared to currents measured in the absence of bestrophin-1. In the presence of the β3-subunit, which is not expressed in the RPE bestrophin-1 did not modulate Ca(V)1.3 activity. Deletion of a cluster of proline-rich motifs in the C-terminus of bestrophin-1 reduced its co-immuno precipitation with the β4-subunit and strongly reduced the Ca(V)1.3 activity. Cells co-expressing bestrophin-1 lacking the proline-rich motifs and Ca(V)1.3 subunits showed less efficient trafficking of bestrophin-1 into the cell membrane. In summary, we conclude that bestrophin-1 modulates L-type channels of the RPE via proline-rich motif-dependent interaction with β4-subunits. A disturbed interaction reduces the currents of the Ca(V)1.3 subunits. This mechanism could open new ways to understand changes in the patient's electro-oculogram and functional alterations of the RPE leading to retinal degeneration.