ABSTRACT
Kosinski-Collins,M.S., Flaugh, S.L. and King , J. (2004) Protein Science, in press.
Probing folding and fluorescence quenching in Human gD Crystallin Greek key domains using Triple Tryptophan mutant proteins
Human gamma-D crystallin (HgD-Crys), a major component of the human eye lens, is a 173 residue, primarily b-sheet protein, associated with juvenile and mature-onset cataracts. HgD-Crys has four tryptophans, with two in each of the homologous Greek key domains, which are conserved throughout the g-crystallin family. HgD-Crys exhibits native-state fluorescence quenching, despite the absence of ligands or cofactors. The tryptophan absorption and fluorescence quenching may influence the lens response to ultraviolet light radiation or the protection of the retina from ambient ultraviolet damage.
To provide fluorescence reporters for each quadrant of the protein, triple mutants each containing three tryptophan to phenylalanine substitutions and one native tryptophan have been constructed and expressed. Trp42-only and Trp130-only exhibited fluorescence quenching between the native and denatured states typical of globular proteins, while Trp68-only and Trp156-only retained the quenching pattern of wild-type HgD-Crys. The 3-D structure of HgD-Crys shows Tyr/Tyr/His aromatic cages surrounding Trp68 and Trp156 that may be the source of the native-state quenching.
During equilibrium refolding/unfolding at 37°C, the tryptophan fluorescence signals indicated that domain I (W42-only and W68-only) unfolded at lower concentrations of GdnHCl than domain II (W130-only and W156-only). Kinetic analysis of both the unfolding and refolding of the triple mutant tryptophan proteins identified an intermediate along the HgD-Crys folding pathway with domain I unfolded and domain II intact. This species is a candidate for the partially folded intermediate in the in vitro aggregation pathway of HgD-Crys.
