Cross-validating FRAP and FCS to quantify the impact of photobleaching on in vivo binding estimates.
Binding can now be quantified in live cells, but the accuracy of such measurements remains uncertain. To address this uncertainty, we compare fluorescence recovery after photobleaching (FRAP) and fluorescence correlation spectroscopy (FCS) measurements of the binding kinetics of a transcription factor, the glucocorticoid receptor, in the nuclei of live cells. We find that the binding residence time measured by FRAP is 15 times longer than that obtained by FCS. We show that this discrepancy is not likely due to the significant differences in concentrations typically used for FRAP and FCS, nor is it likely due to spatial heterogeneity of the nucleus, improper calibration of the FCS focal volume, or the intentional FRAP photobleach. Instead, our data indicate that photobleaching of bound molecules in FCS is mainly responsible. When this effect is minimized, FRAP and FCS measurements nearly agree, although cross-validation by other approaches is now required to rule out mutual errors. Our results demonstrate the necessity of a photobleach correction for FCS measurements of GFP-tagged molecules that are bound for >0.25 s, and represent an important step forward in establishing a gold standard for in vivo binding measurements.
National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA.
Timothy J
TJ
Stasevich
Florian
F
Mueller
Ariel
A
Michelman-Ribeiro
Tilman
T
Rosales
Jay R
JR
Knutson
James G
JG
McNally
D000818:Animals; D055592:Biophysical Phenomena; D045744:Cell Line, Tumor; D055459:Facilitated Diffusion; D036681:Fluorescence Recovery After Photobleaching / Q000379:methods*; D005456:Fluorescent Dyes; D049452:Green Fluorescent Proteins / Q000378:metabolism; D051379:Mice; D038761:Photobleaching; D011485:Protein Binding; D011965:Receptors, Glucocorticoid / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013050:Spectrometry, Fluorescence / Q000379:methods*
D005456:Fluorescent Dyes; D011965:Receptors, Glucocorticoid; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins