Lifetime Regulation of G Protein–Effector Complex: Emerging Importance of RGS Proteins

cGMP. The extremely high amplification of rod phototransduction—universally enabling vertebrate rods to achieve single-photon signaling reliability—is achieved primarily by the cascaded action of the two enzymes R* and PDE*. Vadim Y. Arshavsky* and Edward N. Pugh, Jr.†‡ *Howe Laboratory of Ophthalmology Massachusetts Eye and Ear Infirmary Harvard Medical School Boston, Massachusetts 02114 †Departments of Psychology and Ophthalmology The G protein cascade theory of rod phototransduction faced a deep puzzle in the late 1980s. The molecular University of Pennsylvania Philadelphia, Pennsylvania 19104 theory postulated that the hydrolysis of the terminal phosphate of Gat–GTP was a requisite step in the inactivation of the cascade, but the numbers were way, way With their report in this issue of Neuron on RGS9, a rod off the mark. The responses of amphibian rods to dim photoreceptor GTPase activating protein (GAP), He et flashes at room temperature were known to reach their al. (1998) open a new chapter in the encyclopedic saga response peak in z1 s and subsequently recover with of G protein signal transduction, in the volume on the a terminal time constant of no more than 2 s, while the rod phototransduction cascade. dim flash responses of mammalian rods were found to Vertebrate rod phototransduction (see Figure 1) is inibe nearly ten times faster, peaking in z150 ms and tiated when rhodopsin, a member of the superfamily recovering with a terminal time constant of z200 ms. of seven-helix G protein–coupled receptors, captures a Also in the 1980s, numerous studies of transducin– photon; the covalently preattached ligand of rhodopsin, GTPase activity reported turnover numbers of 1–2 min, 11-cis retinal, is then isomerized to its all trans form, nearly 100-fold too slow to account for the physiologicausing rhodopsin to undergo a conformational change cally measured responses. An unrealistic patch of the and thus enter its enzymatically active signaling state, theory was proposed—namely, that the decline in Ca R*. R* catalyzes GTP/GDP exchange on the a subunit that accompanies the rod light response might accelerof the rod’s heterotrimeric G protein, transducin. The ate guanylyl cyclase activity in such a way as to mask activated moiety of transducin, Gat–GTP, carries the a long-lived activation of PDE by slowly decaying Gat– signal forward to the tertiary effector protein cGMP GTP. Physiological experiments rejected the latter hyphosphodiesterase (PDE) by binding to and relieving an pothesis, showing that cyclase activation in situ was modinhibitory constraint imposed by the g subunit of the est and brief at low flash intensities—certainly inadequate PDE; the catalytic subunits of PDE thus enter their active to bridge the chasm between the biochemically deterstate, PDE*. The first three steps of the phototransducmined GTPase turnover numbers and the kinetics of the tion cascade occur in or in tight association with the dim flash responses of rods. A gauntlet was cast. lamellar membranes of the rod outer segment. In the Several laboratories engaged in photoreceptor biofourth step, PDE*s catalyze the hydrolysis of the cytochemistry took up the challenge and found the rate of plasmic messenger cGMP, which in the resting state transducin GTPase in concentrated suspensions of rod holds open cGMP-activated channels in the rod plasma outer segment membranes to be much higher than that membrane. The channels close as the cytoplasmic cGMP observed in reconstituted systems (Arshavsky et al., 1989, and references therein). These studies provided concentration declines, and they lose their bound