Peptides, antibodies, and. FRET on beads in flow cytometry: A model systemusing fluoresceinated and biotinylated beta-endorphin

Background: Particulate surfaces such as beds are routinely used as platfor ms for molecular assembly for fundamental and practical applications in flo w cytometry. Molecular assembly is transduced as the direct analysis of flu orescence, or as a result of fluorescence resonance energy transfer. Bindin g of fluorescent ligands to beads sometimes alters their emission yield rel ative to the unbound ligands. Characterizing the physical basis of factors that regulate the fluorescence yield of bound fluorophores (on beads) is a necessary step toward their rational use as mediators of numerous fluoresce nce based applications. Methods: We have examined the binding between two biotinylated and fluoresc einated beta-endorphin peptides and commercial streptavidin beads using flo w cytometric analysis. We have analyzed the assembly between a specific mon oclonal antibody and an endorphin. peptide in solution using resonance ener gy transfer and compared the results on beads in flow cytometry using stead y-state and time-resolved fluorescence. Results;: We have defined conditions for binding biotinylated and fluoresce inated endorphin peptides to beads. These measurements suggest that the pep tide structure can influence both the intensity of fluorescence and the mod e of peptide binding on the bead surface. We have defined conditions for bi nding antibody to the bead using biotinylated protein A. We compared and co ntrasted the interactions between the fluoresceinated endorphin peptide and the rhodamine- labeled antibody. In solution rye measure a K-d of <38 nM b y resonance energy transfer and on beads 22 nM. Discussion: Some issues important to the modular assembly of a fluorescence resonance energy transfer (FRET) based sensing scheme have been resolved. The affinity of peptides used herein is a function of their solubility in w ater, and the emission intensity of the bound species depends on the separa tion distance between the fluorescein and the biotin moiety. This is due to the quasi-specific quenching interaction between the fluorescein and a pro ximal binding pocket of streptavidin. Detection of antibodies in solution a nd on beads either by FRET or capture of fluorescent ligands by dark antibo dies subsequently enables the determination of K-d values, which indicate a greement between solution and flow cytometric determinations. (C) 1999 Wile y-Liss, Inc.