Suramin Affects Coupling of Rhodopsin to Transducin

Abstract

Suramin, a polysulfonated naphthylurea, is under investigation for the treatment of several cancers. It interferes with signal transduction through G s, G i, and G o, but structural and kinetic aspects of the molecular mechanism are not well understood. Here, we have investigated the influence of suramin on coupling of bovine rhodopsin to G t, where G-protein activation and receptor structure can be monitored by spectroscopic in vitro assays. G t fluorescence changes in response to rhodopsin-catalyzed nucleotide exchange reveal that suramin inhibits G t activation by slowing down the rate of complex formation between metarhodopsin-II and G t. The metarhodopsin-I/-II photoproduct equilibrium, GTPase activity, and nucleotide uptake by G t are unaffected. Attenuated total reflection Fourier transform infrared spectroscopy shows that the structure of rhodopsin, metarhodopsin-II, and the metarhodopsin-II G t complex is also not altered. Instead, suramin dissociates G t from disk membranes in the dark, whereas metarhodopsin-II G t complexes are stable. Förster resonance energy transfer suggests a suramin-binding site near Trp 207 on the G t α subunit (K d ∼0.5 μM). The kinetic analyses and the structural data are consistent with a specific perturbation by suramin of the membrane attachment site on G t α . Disruption of membrane anchoring may contribute to some of the effects of suramin exerted on other G-proteins.