VISUAL SIGNALING Excess Gβ Suppresses Spontaneous Activity

Elia et al. explored the mechanism by which Drosophila melanogaster photoreceptor cells maintain a low spontaneous activation rate, thereby allowing these cells to detect single photons of light. Using flies hypomorphic for Gβ_e and Gα_q, the authors determined that decreased abundance of Gβ_e did not cause a decrease in the abundance of Gα_q and vice versa. However, decreased abundance of Gβ_e decreased the fraction of Gα_q attached to the plasma membrane and targeted to the phototransduction compartment, the rhabdomere. Nearly complete loss of Gα_q prevented Gβ from binding to the plasma membrane but did not disrupt targeting to the rhabdomere. Gβ_e was present at a concentration about 2.5 times that of Gα_q, and the excess was present in the cytosol in the rhabdomere; approximately equal amounts of Gα_q and Gβ_e were present in the membrane of dark-adapted flies. The hypomorphic Gβ_e alleles showed selective loss of the soluble Gβ_e pool. Whole-cell patch clamp analysis of the photoreceptor cells showed that cells with decreased Gβ_e exhibited higher spontaneous inward currents. In flies in which both Gβ_e and Gα_q were decreased--thus restoring the proper ratio of Gβ_e to Gα_q--spontaneous activity of the photoreceptor cells was similar to that of wild-type cells. Thus, excess Gβ_e in the cytosol of the rhabdomere interacting with Gα_q, which translocates partially from the membrane to the cytosol during stimulation by light, appears to control the spontaneous activity of these cells.

N. Elia, S. Frechter, Y. Gedi, B. Minke, Z. Selinger, Excess of Gβ_e over Gqα_e in vivo prevents dark, spontaneous activity of Drosophila photoreceptors. J. Cell Biol. 171, 517-526 (2005). [Abstract] [Full Text]

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