Physical Evidence of the Coupling of Solubilized 5-HT1A Binding Sites with G Regulatory Proteins

Previous investigations (El Mestikawy et al., J Neurochem 51: 1031-1040, 1988) have shown that 5-HT1A binding sites (R[5-HT1A]) solubilized by CHAPS from rat hippocampal membranes can be modulated by guanine nucleotides, as expected from their solubilization together with associated G regulatory proteins (G). Studies of the hydrodynamic properties of solubilized R[5-HT1A] have been presently carried out in order to assess in a more direct way the presence of R[5-HT1A]-G complexes in the soluble extract. Under control conditions, the sedimentation of a CHAPS extract from hippocampal membranes through a 5-30% sucrose gradient (200,000 g, 17 hr, 4 degrees) gave two maxima of [3H]8-OH-DPAT binding activity corresponding to sedimentation coefficients of 8.0 S and 10.0 S, respectively. Running the gradient in the presence of 1 microM GTP revealed a significant reduction of the 10.0 S peak, as expected from the loss of material (probably a G protein) normally associated with R[5-HT1A]. Conversely, attempts to prevent the dissociation of R[5-HT1A]-G by treatment of CHAPS soluble hippocampal extracts with the cross-linking reagent disuccinimidyl suberate (0.1 mM) resulted in a significant increase (+70%) in [3H]8-OH-DPAT binding activity associated with the appearance of a new sedimenting material with a higher coefficient (16.5 S). Furthermore, [3H]8-OH-DPAT binding became almost completely insensitive to guanine nucleotides as expected from the irreversible coupling by disuccinimidyl suberate of R[5-HT1A] with G protein(s). WGA-agarose chromatography of CHAPS soluble hippocampal extract supplemented with GTP allowed the physical separation of R[5-HT1A] from the bulk of G proteins, and a concomitant decrease of [3H]8-OH-DPAT high affinity binding capacity. Partial recovery of the latter could be achieved by reconstituting R[5-HT1A]-G complexes upon the addition of a mixture of pure bovine Gi + Go to G-deprived soluble extracts. Finally in vivo treatment with Pertussis toxin (5 micrograms intracerebroventricularly, 48 hr before killing) resulted in a significant reduction of the specific binding of [3H]8-OH-DPAT (-36%) to hippocampal membranes and corresponding CHAPS soluble extracts, and a marked decrease in the inhibitory effect of GppNHp. Accordingly the G protein associated with R[5-HT1A] belongs probably to the Gi or Go families.