Recombinant CyPet–SUMO1 and YPet–Ubc9 proteins were mixed and diluted with phosphate buffered saline (PBS) to a total volume of 30 μL. The final concentration of CyPet–SUMO1 was fixed to 1 μM and the final concentration of YPet–Ubc9 varied from 0 to 4 μM. The mixtures were transferred into a 384-well plate (Falcon) and the fluorescence emission spectrum of each well was measured with a fluorescence multi-well plate reader (Molecular Devices, FlexstationII384). Two excitation wavelengths were used: 414 nm to excite CyPet, and 475 nm to excite YPet. Excited at 414 nm, CyPet has an emission peak at 475 nm (FLDD) (see Fig. 2 ). With FRET, another emission peak at 530 nm (Emtotal) can be observed which results from the energy transferred from CyPet to YPet. When the mixture is excited at 475 nm, an emission peak at 530 nm (FLAA) can be observed which is from the direct excitation of YPet but not CyPet.![]()
When a mixture of CyPet–SUMO1 and YPet–Ubc9 recombinant proteins was excited at 414 nm, the emission intensity at 530 nm was consisted of three components: the direct emission of CyPet, the sensitized emission of YPet and the direct emission of YPet. Because the sensitized emission from YPet–Ubc9 (EmFRET) is proportional to the amount of YPet–Ubc9 bound to CyPet–SUMO1 ([YPetUbc9]bound), we can convert the concentration of YPet–Ubc9 in both free ([YPetUbc9]free) and bound ([YPetUbc9]bound) forms to functions of EmFRET. Therefore, we can derive the relationship between EmFRET and the total concentration of YPet–Ubc9 ([YPetUbc9]total), and calculate the Kd between CyPet–SUMO1 and YPet–Ubc9 based on the algorithm we developed. Data were processed in Prism 5 (GraphPad Software).
Design and detection of high sensitive FRET-based detection for protein interactions in sumoylation conjugation cascade. (a) The diagram of FRET-based detection of SUMO1 and its E2 ligase, Ubc9, interaction. (b) Emission spectra of protein mixtures with [CyPet–SUMO1] fixed as 1 μM and [YPet–Ubc9] increased from 0 to 7.5 μM. Excitation wavelength is 414 nm