Excess Gβγ in normal (left) rhabdomeres silences Gα (black). Weak Gβγ mutants (middle) have enough Ga to create spontaneous activity; strong Gβγ mutants (right) do not.

On 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...

The Rockefeller University Press
2005
The Rockefeller University Press
2005
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