Quantification of ERK kinase activity in biological samples using differential sensing.

The understanding of complex biological systems requires an ability to evaluate interacting networks of genes, proteins, and cellular reactions. Enabling technologies that support the rapid quantification of these networks will facilitate the development of biological models, help to identify treatment targets, and assess treatment plans. The biochemical process of protein phosphorylation, which underlies almost all aspects of cell signaling, is typically evaluated by immunoblotting procedures (western blot), or more recently proteomics procedures, which provide qualitative estimates of the concentration of proteins and their modifications in cells. However, protein modifications are difficult to correlate with activity, and while immunoblotting and proteomics approaches have the potential to be quantitative, they require a complex series of steps which diminish reproducibility. Here, a complementary approach is presented, which allows for the rapid quantification of a protein kinase activity in cell lysates and tumor samples. Using the activity of cellular ERK (extracellular signal-regulated kinase) as a test case, an array sensing approach, that utilizes a library of differential peptide-based biosensors and chemometric tools, was used to rapidly quantify nanograms of active ERK in micrograms of unfractionated cell lysates and tumor extracts. This approach has the potential both for high-throughput, and for quantifying the activities of multiple protein kinases in a single biological sample. The critical advantages of this differential sensing approach over others are that it removes the need for the addition of exogenous inhibitors to suppress the activities of major off-target kinases and allows us to quantitate the amount of active kinase in tested samples rather than measuring the changes in its activity upon induction or inhibition.