Pii: S0962-8924(00)01741-4

phatases and other players in signal transduction relocate to membranes, cytoskeletal structures, scaffolding proteins or organelles1–3. Here we take receptor tyrosine kinase (RTK) signalling as our main example (Fig. 1). Stimulation of RTKs is linked to the activation of mitogen-activated protein kinase (MAPK) cascades through a cytoplasmic protein Sos (a homologue of the Drosophila melanogaster ‘Son of sevenless’ protein) and the small GTP-hydrolysing protein Ras, anchored to the cell membrane4. Sos is a GDP/GTP exchange factor that catalyses the conversion of inactive Ras (i.e. its GDP-bound form) to active Ras (its GTP-bound form). The adaptor protein Grb2 (growth-factor-receptor-binding protein 2) mediates the binding of Sos to activated RTKs, such as the epidermal growth factor receptor (EGFR). Grb2 binds to the activated EGFR directly or through another adaptor protein, tyrosine-phosphorylated Shc (src homology and collagen domain protein). The EGFR does not phosphorylate Sos, nor does the catalytic activity of Sos towards Ras change upon Sos binding to the receptor5. When Sos is recruited to the membrane by activated EGFR, Sos can interact with the membrane polyphosphoinositides through an N-terminal pleckstrin homology (PH) domain. This interaction pattern raises a number of questions about the role of the plasma membrane relocation in signal transduction. Why should the Grb2–Sos complex bind to the membrane receptor if Sos catalytic activity is not activated by the receptor? What prevents direct interaction of cytosolic Sos with the membrane-bound Ras from activating the latter? Why is Ras anchored to the membrane? Should anchoring itself be a regulatory event? What is essentially different in the activated versus the nonactivated RTK? It has been proposed that the recruitment of Sos to the proximity of the membranebound Ras is a key feature in the activation of Ras by phosphorylated EGFR6–8. But what does recruitment mean? If it means that Sos is first bound to the EGFR and then moves to Ras by two-dimensional diffusion, then why should this accelerate signal transduction? Cytosolic Sos still requires the same amount of time to reach the EGFR. Binding to EGFR would slow down its diffusion unless Sos dissociated again, but then Sos would escape back to the cytosol rather than bind to Ras. To clarify the effect of membrane localization, we consider two extremes. When two protein molecules form a productive complex (i.e. transduce the signal) after each diffusive encounter, the signal-transduction process is ‘diffusion-limited’. If only a small fraction of the collisions leads to binding that lasts long enough to transfer the information, the signal transduction is ‘reaction-limited’. In this case, the reaction rate is controlled by the alignment of reactive patches in the correct orientation or by the intrinsic chemical transformation rather than by the Brownian collisions of the molecules. The two protein molecules then associate and dissociate several times before signal transduction takes place. We will now analyse the consequences of the membrane translocation for diffusionand reaction-limited signal transduction. Does membrane localization enhance diffusionlimited signal transduction? Adam and Delbrück suggested that the reduction in dimensionality might enhance reaction rates between solutes that bind to membranes and membranebound species9; the solutes should not get lost by wandering off into the bulk phase. The relevance and magnitude of this enhancement has been studied extensively in various biological systems10–12. Conservative estimates can be made of the time taken by signal transduction proteins in the cell FO R U M