An efficient method for FITC-labelling of proteins using tandem affinity purification.

Fluorescence-based assays are extremely diverse, sensitive and robust experimental methods for investigating the conformational changes, enzyme kinetics, dynamics and molecular interactions. A prerequisite for most of these experimental approaches is to label the protein of interest with one or more extrinsic fluorophores with desired photophysical properties. Fluorescein isothiocyanate (FITC), due to its high quantum efficiency and conjugate stability is most widely used fluorescence labelling reagent for such experimental approaches. However, the bottlenecks in this labelling reaction is requirement of high protein concentration, maintenance of protein stability during the labelling process as well as high background fluorescence due to ineffective removal of unreacted FITC, prior to fluorescence studies. Therefore, to overcome these inadequacies or limitations, we have modified the existing protocol by introducing tandem affinity purification (TAP) tags at the N- and C-termini of the target protein. Using this modified method, we have efficiently labelled target protein with significant decrease in precipitation, degradation and background fluorescence of unreacted FITC. This facile and rapid technique may also be used as a basis for labelling procedures with other fluorophores and hence has a broad application in spectroscopic studies.