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D crystallin influencing folding and stability
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
(RECEIVED March 14, 2005; FINAL REVISION April 29, 2005; ACCEPTED May 6, 2005)
Human 
D crystallin (HD-Crys) is a two domain, 
-sheet eye lens protein that must remain soluble throughout life for lens transparency. Single amino acid substitutions of HD-Crys are associated with juvenile-onset cataracts. Features of the interface between the two domains conserved among -crystallins are a central six-residue hydrophobic cluster, and two pairs of interacting residues flanking the cluster. In HD-Crys these pairs are Gln54/Gln143 and Arg79/Met147. We previously reported contributions of the hydrophobic cluster residues to protein stability. In this study alanine substitutions of the flanking residue pairs were constructed and analyzed. Equilibrium unfolding/refolding experiments at 37°C revealed a plateau in the unfolding/refolding transitions, suggesting population of a partially folded intermediate with a folded C-terminal domain (C-td) and unfolded N-terminal domain (N-td). The N-td was destabilized by substituting residues from both domains. In contrast, the C-td was not significantly affected by substitutions of either domain. Refolding rates of the N-td were significantly decreased for mutants of either domain. In contrast, refolding rates of the C-td were similar to wild type for mutants of either domain. Therefore, domain interface residues of the folded C-td probably nucleate refolding of the N-td. We suggest that these residues stabilize the native state by shielding the central hydrophobic cluster from solvent. Glutamine and methionine side chains are among the residues covalently damaged in aged and cataractous lenses. Such damage may generate partially unfolded, aggregation- prone conformations of HD-Crys that could be significant in cataract.
Keywords: human D crystallin; domain interface; partially folded intermediate; cataract; equilibrium unfolding/refolding transitions
Article and publication are at http://www.proteinscience.org/cgi/doi/10.1110/ps.051460505.
Reprint requests to: Jonathan King, Department of Biology, Massachusetts Institute of Technology, Building 68, Room 330, 31 Ames Street, Cambridge, MA 02139, USA; e-mail: jaking{at}mit.edu; fax: (617) 252-1843.
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