Transducin T , the light-activated GTPase

Inhibition of adenylyl cyclase by low (/aM) concentrations of GTP was first reported in 1973-4 [57-59]. Further studies strongly suggested that hormones that attenuate adenylyl cyclase activity do so via a GTP-dependent step, akin to that intervening between stimulatory receptors and adenylyl cyclase (for reviews see Refs. 

The existence of a molecule coupling inhibitory hormonal receptors to ad-enylyl cyclase that is distinct from Gs was demonstrated in 1983 with the aid of PTX [62] and the eye cell line [23]. The purification of PTX-substrates of subunit composition a/3, later revised to ajS-y [5], was also reported in 1983 [63,64]. At that time, the only cellular function known to be blocked by PTX treatment was hormonal inhibition of adenylyl cyclase. The purified proteins, one from liver, with a subunit of 41 kDa [63], the other from human red blood cells (hRBCs), with a subunit of 40 kDa [64], were eventually named Gj (N(). 

In contrast to Gs and T, purified Gj has failed to work well in reconstitution assays designed to test for its ability to inhibit adenylyl cyclase in a manner predicted by the mode of action of Gs or T [69,71,72].Thus, a definitive functional assay to confirm the identity of the purified proteins may be missing. 

For discussion purposes, we shall refer to the functionally defined inhibitory G as Gj and the purified 40-41000 Da PTX substrates, as Gj . 

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