Shedding light on local kinase activation

Intracellular protein kinases relay information that originates from diverse chemical and ionic signals received at the cell surface, phosphorylating their substrates to elicit a variety of responses. A relatively small group of second messenger molecules, and the kinases they activate, function in a plethora of different signaling pathways, posing the question of how the specificity of the original signal is not lost in transmission. One answer to this appears to be the routing of signals with spatial fidelity and tight temporal control, through signaling complexes brought together on protein scaffolds. As we illustrate in this article, direct insights into local patterns of kinase activation have been made possible by the ingenious development of genetically encoded kinase activity reporters. These biosensors translate the language of cell signaling, as it occurs in real time, into observable bursts of light.Nature has an abundance of fluorescent proteins that absorb and emit light [1]. Since these fluorescent proteins, or chromophores, have unique excitation/emission properties, each can be independently detected. Moreover, they can be paired in such a way that, when brought into close proximity, it is possible to detect energy transfer between them if the emission spectrum of one - the donor - overlaps with the absorption spectrum of the other - the acceptor. The elegant but simple logic behind 'biosensors' is to design a light-activated protein so that it responds to an altered biochemical parameter - whether it be phosphorylation, second messenger synthesis or recruitment of a binding partner - with detectable changes in the light spectrum emitted. Additionally, other elements of the biosensor are designed so that a change in the biological parameter of interest induces an intramolecular change, and thereby alters fluorescence resonance energy transfer (FRET) from the donor to the acceptor chromophore (Figure 1). A common donor-acceptor pair comprises monomeri