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Through the course of evolution, biological systems have developed several conserved mechanisms by which cells can rapidly modify their responsiveness to extra-cellular stimuli. These include biochemical and allosteric alterations in the properties of key signalling molecules, such as ion channels, transmembrane receptors and enzymes. Several such modifications have been identi-fied as being particularly important for various forms of synaptic plasticity. For example, activation/inhibition of protein kinases and phosphatases1–3 and the consequent phosphorylation/dephosphorylation of critical synaptic substrates, such as receptors4, are crucial for activity-dependent changes of synaptic efficacy. Owing to its key role in modulating the responsive-ness of a wide range of cell types, the regulated endo