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Probing folding and fluorescence quenching in human D crystallin Greek key domains using triple tryptophan mutant proteins

Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

(RECEIVED January 27, 2004; FINAL REVISION May 3, 2004; ACCEPTED May 8, 2004)

Human D crystallin (HD-Crys), a major component of the human eye lens, is a 173-residue, primarily -sheet protein, associated with juvenile and mature-onset cataracts. HD-Crys has four tryptophans, with two in each of the homologous Greek key domains, which are conserved throughout the -crystallin family. HD-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 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. Trp 42-only and Trp 130-only exhibited fluorescence quenching between the native and denatured states typical of globular proteins, whereas Trp 68-only and Trp 156-only retained the anomalous quenching pattern of wild-type HD-Crys. The three-dimensional structure of HD-Crys shows Tyr/Tyr/His aromatic cages surrounding Trp 68 and Trp 156 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 HD-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 HD-Crys.

Keywords: Human D Crystallin; tryptophan; fluorescence; protein folding; folding intermediates; fluorescence quenching

Reprint requests to: Jonathan King, Department of Biology, Massachu-setts Institute of Technology, Building 68, Room 330, 31 Ames Street, Cambridge, MA 02139, USA; e-mail: jaking{at}mit.edu; fax: (617) 252-1843.

Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.04627004.

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