Proximity of lectin receptors on the cell surface measured by fluorescence energy transfer in a flow system.

Molecules of the lectin concanavalin A have been labeled separately with the fluorescein and rhodamine chromophores and jointly bound to the surface of transformed Friend erythroleukemia cells. The two dyes constitute an ideal donor-acceptor pair for fluorescence resonance energy transfer thereby permitting the determination of the proximity relationships between bound ligand molecules and the corresponding surface receptors. The transfer efficiency at saturation (about 57%) was measured in a multiparameter flow system using laser excitation at 488 nm and detection of fluorescein and rhodamine emission intensities as well as the emission anisotropy of the rhodamine fluorescence for each cell. The degree of energy transfer was estimated from the quenching of donor emission, the sensitization of acceptor emission, and the depolarization of acceptor fluorescence. The system has been modeled according to a formalism developed by Gennis and Cantor (Biochemistry 11: 2509, 1972). We estimate the separation between the surfaces of bound lectin molecules at saturation to be 0-40 A, a range possibly characteristic for micropatches induced by ligand binding.