Mag-Indo-1 as a potential reporter of the 3D conformation of protein sub-domains.

Mag-indo-1 is a well known fluorescent probe. Magnesium complexation results in a shift of the emission fluorescence spectrum from 480nm to 417nm with an intensity proportional to the magnesium concentration in the range 0.6 to 30 mM. Although designed as a specific magnesium chelator, Magindo-1 is also able to bind calcium and zinc. All these cationic interactions induce the same spectral shift but the fluorescence intensity and the dissociation constant are dependent of the nature of the cation. Furthermore Mag-indo-1 can also bind proteins through a specific interaction with some histidin residues. That interaction induces a characteristic spectral shift of the emission fluorescence spectra from 480 to 457nM. All these properties suggest that Mag-indo-1 could be used to study the protein-cation binding. Emission and synchronous fluorescence techniques have been used to monitor that interaction with proteins such as bovin serum albumin, human serum albumin, turkey white egg lysozyme. Using a method of resolution of complex fluorescence spectra, it has been possible to calculate the number of interaction sites and the correlative dissociation constants. Depending on the nature of the protein a quenching of the natural fluorescence of the protein was observed, associated with an energy transfer from some tryptophan(s) to Mag-indo-1. All these data were tentatively correlated with the available information on the 3D conformation of the proteins. These results suggest that Mag-indo-1 could be used as an intramolecular fluorescent ruler to monitor the changes in 3D conformation of specific subdomains of proteins.