No significant variations are observed in the fluorescence spectrum obtained after addition to N TAILW518 of a 2 mM solution of an irrelevant protein (SARS virus, unclassified protein 5) of similar size and devoid of tryptophan residues (data not shown). After plotting the relative fluorescence intensity increase as a function of the XD concentration (Fig. 6C) , an apparent constant equilibrium dissociation (K Dapp ) value of 133 6 32 mM is derived. This value is in good agreement with the value obtained by surface plasmon resonance studies with wt N TAIL . Although the environment of Trp 518 remains mostly polar upon binding to XD, the observed increase in the fluorescence intensity indicates that the chemical environment of Trp 518 is affected, thus further suggesting that the C terminus of N TAIL interacts with XD.