STEPWISE PHOSPHORYLATION MECHANISMS AND SIGNAL TRANSMISSION WITHIN A LIGAND-RECEPTOR-G-ALPHA-BETA-GAMMA-PROTEIN-COMPLEX

An extended alpha-helical structural model of a G-protein-coupled rece ptor has been combined with a G-protein structure to assess the ternar y complex's potential to function as a GTP synthase. The establishment of the structural constraints between the G-protein and the receptor model has been based on the likelihood of a proton signalling mechanis m existing within the ternary complex which can conduct a proton from the interior of the receptor to a phosphorylated histidine residue on the G beta-subunit. The candidate histidine residue is beta 183. The e arlier development of the receptor's action as a ligand-activated tran s-membrane proton transducer is concluded to be potentially more exten sive in the extended receptor structure, the receptor's proton signall ing mechanism being based on a Tyr-Arg-Tyr-Arg-Tyr proton shuttle pent ad. Further proton shuttle mechanisms can exist in the G alpha,G beta subunit interface. The final Tyr-Arg-Tyr shuttle triad exhibits an oxy gen-oxygen interatomic distance of some 12.0 Angstrom in the G-protein crystal structure consistent with the distance predicted previously w ithin a receptor proton shuttle triad. The arrival of a proton on the acid labile histidine phosphoramidate would enable release of the phos phate group and, through an expected S(N)2 reaction, allow transfer to the neighbouring arginine residue alpha 201 (G alpha t numbering) wit h the formation of a high energy phosphono-arginine intermediate. Subs equent transport of the phosphate group to the GDP phosphorylation sit e involving the known movement of the G alpha alpha 2-helix at the G a lpha-G beta subunit interface would then complete the synthesis throug h this stepwise phosphorylation. In the general context of cell signal ling under more active conditions, the direct phosphorylation mechanis m is consistent with the cell requiring more energy input than can be obtained by a GDP/GTP exchange process, the explicit catalytic phospho rylation mechanism being faster than the generally accepted exchange m echanism. (C) 1998 Elsevier Science B.V. All rights reserved.