G-protein, a signal coupling protein in invertebrate photoreceptors, was characterized by toxin-labeling and antibody-binding experiments. A 41 kDa protein of octopus photoreceptors is specifically ADP-ribosylated by pertussis toxin. Labeling is maximal in the dark in the presence of GDP/B, as observed with vertebrate transducin. Furthermore, an antiserum prepared against the /3-(35 kDa) and y-(8 kDa) subunits of bovine transducin cross-reacts with a 36 kDa protein in octopus photoreceptors. These results indicate that the c+ and p-subunits of octopus photoreceptor G-protein are akin to those of vertebrate transducin and other G-proteins. G-protein Transducin

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Two photocycles due to two different pigments were found in membrane vesicles of a bacteriorhodopsin-free mutant of Halobacterium halobium. A pigment absorbing approximately 590 nm halorhodopsin (HR) underwent a faster photocycle with a phototransient at approximately 490 nm (half-time of decay, tau 1/2 = 10 ms). Another third rhodopsinlike pigment (TR) absorbing approximately 580 nm underwent a slower photocycle accompanying a phototransient absorbing below 410 nm (tau 1/2 = 0.8s). The photocycles were measured under various conditions of temperature, NaCl concentration, pH, and in the presence of cholate. All results obtained support the notion that the two photocycles are independent of each other, and the fast or the slow cycle can be abolished after these treatments. At alkaline pH, the wavelength of maximum absorbance of both pigments shifted to blue, but the magnitude of the shift of the pigment undergoing the slow photocycle was much greater than the other. The ratio of the content of the two pigments varies among bacteriorhodopsin-free mutants.

The cytoplasmic membranes of Halobacterium halobium contain at least three retinal pigments: bacteriorhodopsin (bR), halorhodopsin (hR), and a third rhodopsinlike pigment (tR). The amplitudes of the phototransient in the photolysis of hR and tR were measured in various salt solutions. Halogen ion (except fluoride) was required to retain the photocycle of hR. Parallels between the amplitude of the phototransient of hR and the magnitude of the photo-induced tetraphenylphosphonium (TPP+) uptake suggests that hR is a light-driven halogen pump, which supports the hypothesis by Schobert and Lanyi (J. Biol. Chem., 1982, 257:10306-10313). The order of effectiveness of halogen was Br- greater than Cl- greater than I-. On the other hand, no specific ion was required to retain the photocycle of tR, and tR was concluded to be nonelectrogenic.

The effect of pressure on the dissociation of boric acid, imidazole HCl, and H2PO4− in aqueous buffers has been investigated up to 6 kbars, using as optical indicators p-nitrophenol and Cresol Red. The limiting volume change on ionization at atmospheric pressure (Δ\barV°) and the compressibility change (Δ\barκ°) for these weak acids were obtained. The values of Δ\barV° (cm3 mol−1) and Δ\barκ° (cm3 mol−1 kbar−1) are −10.1 and −0.4 for Cresol Red, −30.2 and −4.3 for boric acid, and −2.4 and −0.6 for imidazole H+.