Published online 31 October 2005. doi:10.1083/jcb1713iti4
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 171, Number 3, 405-405
G proteins live in excess
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 
subunit to exchange its bound GDP for GTP, dissociate from its binding partner, ß
, and initiate downstream signaling. The group now finds that excess ß ensures that is not activated in the dark.
Wild-type photoreceptors had over twofold more ß than and low background activity. Mutants with less ß had much more spontaneous activity. This defect was corrected by simultaneously reducing levels in the mutant (thus restoring the ß excess).
Spontaneous activity was actually higher in cells with moderate rather than extreme reduction in ß. The authors explain this finding by showing that ß was needed to bring to the rhabdomere, from which signals. Thus, in stronger ß mutants, there was less able to signal and hence less spontaneous activity.
The group must now determine how the excess ß limits activity. Perhaps it either accelerates GTP hydrolysis on to block the downstream cascade or prevents the unsolicited exchange of GDP for GTP on . 
Nicole LeBrasseur
lebrasn{at}rockefeller.edu
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Facebook 
Reddit 
Technorati 
Twitter What's this?
Related Article
-
Excess of Gße over Gqe in vivo prevents dark, spontaneous activity of Drosophila photoreceptors
- Natalie Elia, Shahar Frechter, Yinon Gedi, Baruch Minke, and Zvi Selinger
J. Cell Biol. 2005 171: 517-526.
[Abstract]
[Full Text]
[PDF]
