Suramin Affects Coupling of Rhodopsin to Transducin

• Published in: Biophysical journal 2002-02, Vol.82 (2), p.793-802
• Main Authors: Lehmann, Nicole, Krishna Aradhyam, Gopala, Fahmy, Karim
• Format: Article
• Language: English
• Subjects: Animals; Antineoplastic Agents - pharmacology; Binding Sites; Cattle; Cell Cycle; Drugs; GTP Phosphohydrolases - metabolism; Guanosine Triphosphate - metabolism; Kinetics; Light; Protein Binding - drug effects; Proteins; Retina - metabolism; Rhodopsin - chemistry; Rod Cell Outer Segment - metabolism; Side effects; Signal Transduction; Spectrometry, Fluorescence; Spectroscopy, Fourier Transform Infrared; Suramin - pharmacology; Time Factors; Transducin - chemistry; Tryptophan - chemistry
• ISSN: 0006-3495; 1542-0086
• DOI: 10.1016/S0006-3495(02)75441-6

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.