page 517, Elia et al. show how fly photoreceptors achieve their exquisite sensitivity to a single photon of light. The key is not the number of photoreceptor-activating proteins but the ratios of their components.
Photoreceptor sensitivity depends on extremely low levels of spontaneous activity in the dark. This activity, spontaneous or otherwise, depends on a G protein coupled to the rhodopsin receptor. Rhodopsin activation induces the G protein's α subunit to exchange its bound GDP for GTP, dissociate from its binding partner, βγ, and initiate downstream signaling. The group now finds that excess βγ ensures that α is not activated in the dark.
Wild-type photoreceptors had over twofold more βγ than α and low background activity. Mutants with less βγ had much more spontaneous activity. This defect was corrected by simultaneously reducing α levels in the mutant (thus restoring the βγ excess).
Spontaneous activity was actually higher in cells with moderate rather than extreme reduction in βγ. The authors explain this finding by showing that βγ was needed to bring α to the rhabdomere, from which α signals. Thus, in stronger βγ mutants, there was less α able to signal and hence less spontaneous activity.
The group must now determine how the excess βγ limits α activity. Perhaps it either accelerates GTP hydrolysis on α to block the downstream cascade or prevents the unsolicited exchange of GDP for GTP on α.