Adhesion proteins meet receptors: a common theme?

Receptors tyrosine kinases (RTKs) and cell adhesion molecules (CAMs) present on the cell surface sense the surrounding environment and influence the fate of cells. For a long time, it was believed that these molecules were working independently and that the sole binding of a ligand was enough to activate the RTK. It is now apparent that there is, in fact, a very tight connection between RTKs and CAMs and that they work in concert. The CAMs influence the activation, the signaling, or the internalization of the RTKs. Some CAMs have similar functions and are therefore interchangeable. CD44 isoforms exemplify the flexibility of these interactions as they can collaborate with several RTKs and can also be substituted by other CAMs with similar functions. In several instances, CAMs not only control the activation of the receptor by presenting the ligand but also regulate the downstream signaling by organizing a signalosome complex. Furthermore, the functions of the CAMs can be controlled by the cellular environment and the binding to their ligands.