A genetically encoded biosensor is letting researchers watch enzymes in action. Designed by US scientists, the biosensor can be used to follow protein dephosphorylation inside cells in real time.

Dephosphorylation is controlled by phosphatase enzymes and is an important cellular process involved in signalling pathways. To understand how phosphatases regulate cellular processes in real-time, Jin Zhang and Robert Newman at Johns Hopkins University, Baltimore, created a biosensor sensitive to the phosphatase calcineurin.

The action of a phosphatase (PPase) causes a conformational change in a biosensor which is detected as a change in fluorescence

The new biosensor is based on a DNA construct which is transferred into mammalian cells. Once inside the cells, the DNA is expressed, creating a chimera protein (see figure) comprised of two fluorophores (shown in blue and yellow) separated by a protein-based molecular switch (shown in grey). The molecular switch was designed from a substrate of calcineurin and changes conformation upon dephosphorylation by the phosphatase. This conformation change alters the separation between the two fluorophores and this can be detected by fluorescence resonance energy transfer. This allows the researchers to study where and when calcineurin dephosphorylates proteins in the cell.

'This method is a general design or approach for generating genetically encoded biosensors,' explained Zhang. There are alternative methods for studying dephosphorylation but this approach 'offers better spatial and temporal information,' she added. 

Hongzhi Xie a scientist researching biochemical sensors at Los Alamos National Laboratory, US, welcomed the study, saying 'such a novel biosensor will have broad applications in living cell and deep tissue imaging, kinase activity assays and high-throughput screening for drug discovery.' 

Russell Johnson