A New Mechanism for G Protein Activation

We present a new model, which is mainly based on observations stemming from rod outer segments (ROSs), since in these segments transducin is largely abundant and the system is therefore deal for studying G protein activation. Throughout, we assume that findings for one particular type of G protein present in a certain species and tissue can be used to infer the activation mechanism or G proteins in general. Within the new model, all experimental findings concerning exchange reactions are explained, as well as the observation that phosphorylation reactions regulate G protein functioning.1–4 Also the seemingly contradictory results an interpretations of Vuong and co-workers5–7 and the observed nucleotide levels reported by Robinson and Hagins8 are brought into accordance within this model. Figure 7.1 summarizes our ideas regarding, in our opinion, a more satisfactory mechanism for G protein activation than is provided by the classic model; the new model is discussed step by step in the subsequent subsections.