Fluorescence can be used as a spectroscopic ruler to study and quantify the interactions of cellular components on the molecular level. Fluorescence resonance energy transfer is a process by which a fluorophore (donor) in an excited state may transfer its excitation energy to a neighboring chromophore (acceptor) nonradiatively through dipole-dipole interactions. The usefulness of this technique derives from the fact that the efficiency of the energy transfer process varies as the inverse of the sixth power of the distance separating the donor and acceptor fluorophores, resulting in the ability to measure interactions between cellular components on a scale of 10-50Å . One of the cellular components to be studied is labeled with donor fluorophores and the other cellular component is labeled with acceptor fluorophores. By design, the emission spectra of the donor fluorophores overlap the excitation spectrum of the acceptor fluorophores. When the distance separating the donors and acceptors is less than or equal to 50Å, then energy from the excited donors can non-radiatively transfer to the acceptors. The extent of transfer is related to the distance separating the donor and acceptor fluorophores (and by design, the structures they are attached to).

Lateral segregation of plasma membrane lipids during cell injury is being examined using FRETM. Measurements made using FRETM have provided a potential mechanism to account for the alterations in plasma membrane lipid order and topography attendant with hypoxic injury. A picture has emerged where hypoxia induces the formation of domains composed of identical phospholipids, and these domains become packed tighter and tighter as injury progresses, eventually causing holes or gaps in the membrane to occur leading to loss of the plasma membrane permeability barrier and the onset of irreversible injury. FRET is also being applied to test the hypothesis that growth factor binding results in receptor dimerization which is required for biological activity. These studies are focused on Platelet-derived Growth Factor receptors, as these receptors and their ligands play a major role in tumors of mesenchymal origin and atherosclerosis. The application of green fluorescent proteins (GFP) as donors and/or acceptors is also being explored in this model system. Lastly, FRETM is being employed to study the interaction between high risk HPV E6 protein and the tumor suppressor protein p53. FRETM demonstrated that HVP E6 and p53 were separated by a distance less than 50 Angstroms, and hence are most probably bound together in a complex. This study substantiated in vitro studies showing that E6 can bind p53 protein and indicate that part of the mechanism by which HPV-16/18 could cause cervical neoplasia may be that cytoplasmic HPV E6 binds to p53 protein after its transcription in the cytoplasm, preventing p53 from entering the nucleus where it performs its normal growth (tumor) suppressor function.