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Drosophila melanogaster photoreceptor cells are capable of detecting single photons. This utmost sensitivity is critically dependent on the maintenance of an exceedingly low, dark, spontaneous activity of photoreceptor cells. However, the underlying mechanisms of this hallmark of phototransduction are not fully understood. An analysis of the Drosophila visual heterotrimeric ([alpha][beta][gamma]) Gq protein revealed that wild-type Drosophila flies have about a twofold excess of G[beta] over G[alpha] subunits of the visual Gq protein. Studies of G[[beta].sub.e] mutants in which the excess of G[beta] was genetically eliminated showed dramatic dark, spontaneous activity of the photoreceptor cells, whereas concurrent genetic reduction of the G[alpha] subunit, which restored the excess of G[beta], abolished this effect. These results indicate that an excess of G[beta] over G[alpha] is a strategy used in vivo for the suppression of spontaneous activity, thereby yielding a high signal to noise ratio, which is characteristic of the photoreceptor light response. This mechanism could be relevant to the regulation of G protein signaling in general.

Bacterial GTPase-activating proteins (GAPs) subvert their host's eukaryotic Rho GTPases to their own advantage. Studies of bacterial GAPs extend our understanding of the action of eukaryotic GAPs, provide new tools for studies of cytoskeletal dynamics and offer new targets for anti-bacterial drugs.

Results from homology modeling reveal that the heterotrimeric human G(sub)alpha proteins exhibit a basic, positively charged, structural motif at the N-termini, which influence both membrane affinity and orientation of the G proteins. Data further suggest that the alpha-helical basic motif participates in the membrane-targeting and palmitylation-determining signaling.

The N terminus of the Aeromonas salmonicida ADP-ribosylating toxin AexT displays in vitro GTPase-activating protein (GAP) activity for Rac1, CDC42, and RhoA. HeLa cells transfected with the AexT N terminus exhibit rounding and actin disordering. We propose that the Aeromonas salmonicida AexT toxin is a novel member of the growing family of bacterial RhoGAPs.

A GTPase specific assay was developed to understand the mechanism of transmembrane signaling related to hydrolysis of GTP that resets the active receptor adenylyl cyclase to the inactive site. The results showed that the catecholamine adrenergic agonists stimulated the hydrolysis of adenylyl cyclase activity by facilitating the exchange of bound GDP for free GTP.