Chemical Modification of the Tryptophan Residue in a Recombinant Ca²⁺-ATPase N-domain for Studying Tryptophan-ANS FRET

The sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) is a P-type ATPase that has been crystallized in various conformations. Detailed functional information may nonetheless be obtained from isolated recombinant domains. The engineered (Trp552Leu and Tyr587Trp) recombinant nucleotide-binding domain (N-domain) displays fluorescence quenching upon ligand binding. An extrinsic fluorophore, namely, 8-anilino-1-naphthalene sulfonate (ANS), binds to the nucleotide-binding site via electrostatic and hydrophobic interactions with Arg, His, Ala, Leu, and Phe residues. ANS binding is evidenced by the increase in fluorescence intensity when excited at a wavelength (λ) of 370 nm. However, when excited at λ of 295 nm, the increase in fluorescence intensity seems to be coupled to the quenching of the N-domain intrinsic fluorescence. Fluorescence spectra display a Föster resonance energy transfer (FRET)-like pattern, thereby suggesting the presence of a Trp-ANS FRET pair, which appears to be supported by the short distance (~20 Å) between Tyr587Trp and ANS. This study describes an analysis of the Trp-ANS FRET pair by Trp chemical modification (and fluorescence quenching) that is mediated by N-bromosuccinimide (NBS). In the chemically modified N-domain, ANS fluorescence increased when excited at a λ of 295 nm, similar to when excited at a λ of 370 nm. Hence, the NBS-mediated chemical modification of the Trp residue can be used to probe the absence of FRET between Trp and ANS. In the absence of Trp fluorescence, one should not observe an increase in ANS fluorescence. The chemical modification of Trp residues in proteins by NBS may be useful for examining FRET between Trp residues that are close to the bound ANS. This assay will likely also be useful when using other fluorophores.