Signal-transducing guanine-nucleotide-binding regulatory proteins (G proteins) Enzymes. Creatine kinase (E.C. 2.7.3.2); GTPase (E.C. 3.6.1 .-); nucleoside diphosphokinase (E.C. 2.7.4.6).hormones and neurotransmitters, are composed of three distinct subunits: a guanine-nucleotide-binding CI subunit, a P subunit and a y subunit, the latter two forming a fly dimer under non-denaturing conditions [l -31. Both components, i. e. the cl subunit and the fly dimer, are required for receptorcontrolled activation of G proteins [4-61. However, the exact role of B y subunits, apparently acting in a substoichiometric manner in activation of CI subunits, is not known. The fly dimers of the G protein transducin, found in vertebrate rod outer segment (ROS) membranes, are different from other G protein B y dimers in that apparently only one form of each B and y subunit is present in ROS membranes, while other membranes contain at least two distinct fl subunits, one (P36)

Bovine rod outer segment (ROS) membranes contain in addition to the heterotrimeric G protein transducin, several small GTP-binding proteins (23-27 kDa). Furthermore, these membranes contain two substrate proteins (about 22 and 24 kDa) for botulinurn C3 ADP-ribosyltransferase known to ADP-ribosylate small G proteins in any mammalian cell type studied so far. Most interestingly, P*P]ADP-ribosylation of ROS membrane small G proteins by C3 is regulated by light and guanine nucleotides in a manner similar to pertussis toxin-catalyzed[32P]ADP-ribosy1ation of the a-subunit of transducin.These findings suggest that not only the heterotrimeric G protein transducin but also the C3 substrate small G proteins present in ROS membranes interact with photoexcited rhodopsin and thus contribute to its signalling action.GTP-binding protein; Rhodopsin; Botulinum C3 ADP-ribosyl-transferase; Transducin; (Bovine rod outer segment)

No abstract

The small molecular mass GTP-binding proteins rho A, B and C are targets for ADP-ribosyltransferase activity of the botulinum exoenzyme C3. The possible interaction of recombinant rho A proteins expressed in E. coli with photoexcited rhodopsin was studied by reconstitution with bovine rod outer segment (ROS) membranes depleted of endogenous GTP-binding proteins by treatment with urea. As reported for C3 substrates present in untreated ROS membranes, ADP-ribosylation of recombinant rho A proteins, both normal and Val-14 mutant, by C3 was inhibited when reconstituted with illuminated compared to dark-adapted ROS membranes pretreated with urea. GDP reduced the light-induced inhibition, while GTP [S] and light inhibited ADP-ribosylation of rho A proteins in a synergistic manner.Rho protein; GTP-binding protein; Rhodopsin; Botulinum C3 ADP-ribosyltransferase; Bovine rod outer segment