Mutations in the bestrophin-1 (Best1) gene are linked to several kinds of macular degeneration in both humans and dogs. Although bestrophins have been shown clearly to be Cl⁻ion channels, it is controversial whether Cl⁻channel dysfunction can explain the diseases. It has been suggested that bestrophins are multifunctional proteins: they may regulate voltage-gated Ca²⁺channels in addition to functioning as Cl⁻channels. Here, we show that humanBest1gene (hBest1) differentially modulates Ca_V1.3 (L-type) voltage-gated Ca²⁺channels through association with the Ca_Vβ subunit. In transfected human embryonic kidney 293 cells, hBest1 inhibited Ca_V1.3. Inhibition of Ca_V1.3 was not observed in the absence of the β subunit. Also, the hBest1 C terminus binds to Ca_Vβ subunits, suggesting that the effect of hBest1 was mediated by the Ca_Vβ subunit. The region of hBest1 responsible for the effect was localized to a region (amino acids 330–370) in the cytoplasmic C terminus that contains a predicted src-homology-binding domain that is not present in other bestrophin subtypes. Mutation of Pro³³⁰and Pro³³⁴abolished the effects of hBest1 on Ca_V1.3. The effect was specific to hBest1; it was not observed with mouse Best1 (mBest1), mBest2, or mBest3. Wild-type hBest1 and the disease-causing mutants R92S, G299R, and D312N inhibited Ca_Vcurrents the same amount, whereas the A146K and G222E mutants were less effective. We propose that hBest1 regulates Ca_Vchannels by interacting with the Ca_Vβ subunit and altering channel availability. Our findings reveal a novel function of bestrophin in regulation of Ca_Vchannels and suggest a possible mechanism for the role of hBest1 in macular degeneration.

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