Co-operation — whether at the atomic, molecular or cellular level — is a key concept in science. Within the confines of a cell, different reactions must be tightly regulated. At the very least, this avoids total chaos, but for the most part, such complexity has gone hand-in-hand with evolution. To reap the true benefits, processes have to be co-ordinated and integrated. A fine example of this is provided by Peter J. Cullen and Peter J. Lockyer who, on page 339, discuss how signalling by calcium and Ras should no longer be viewed as separate entities, as Ras activation can be regulated by calcium signalling. Ras is activated at the plasma membrane, whereas calcium is sequestered in many discrete intracellular regions — so, naturally, this interplay is governed by the spatial and temporal distribution of both molecules.
Another molecule that has a distinct subcellular distribution is the adenomatous polyposis coli (APC) protein, and Mariann Bienz (p328) reviews the evidence that each of four separate pools has a specific function. APC is not confined to any one of these pools, and can shuttle between these destinations, although how this is regulated is not yet clear. Perhaps targeting proteins that are not yet characterized influence its diffusion?
