Illuminating the dynamics of intracellular activity with 'active' molecular reporters

Traditionally, fluorescent and luminescent reporter proteins have been used as indicators of gene expression and protein localization. However, insightful mutagenesis and protein engineering strategies have transformed these simple passive reporters into active biological sensors. Molecular reporters are now being designed to alter their intrinsic optical properties in response to specific biomolecular interactions. Applications for these novel biological sensors range from monitoring intracellular pH and ion fluxes to detecting protein-protein interactions and enzymatic activity. The ability to monitor the dynamics of intracellular activity in response to external stimuli can help elucidate the cascade of events involved in complex processes such as mechanotransduction. Here we review some of the approaches used to create these novel biological sensors, including resonance energy transfer (RET) between reporter proteins and protein fragmentation strategies.