Diabetes mellitus due to misfolding of a beta-cell transcription factor: stereospecific frustration of a Schellman motif in HNF-1alpha.

Abstract

Maturity-onset diabetes of the young (MODY3), a monogenic form of type II diabetes mellitus, results most commonly from mutations in hepatocyte nuclear factor 1alpha (HNF-1alpha). Diabetes-associated mutation G20R perturbs the dimerization domain of HNF-1alpha, an intertwined four-helix bundle. In the wild-type structure G20 participates in a Schellman motif to cap an alpha-helix; its dihedral angles lie in the right side of the Ramachandran plot (alpha(L) region; phi 97 degrees). Substitutions G20R and G20A lead to dimeric molten globules of low stability, suggesting that the impaired function of the diabetes-associated transcription factor is due in large part to a main-chain perturbation rather than to specific features of the Arg side-chain. This hypothesis is supported by the enhanced stability of non-standard analogues containing D-Ala or D-Ser at position 20. The crystal structure of the D-Ala20 analogue, determined to a resolution of 1.4 A, is essentially identical to the wild-type structure in the same crystal form. The mean root-mean-square deviation between equivalent C(alpha) atoms (residues 5-28) is 0.3 A; (phi, psi) angles of D-Ala20 are the same as those of G20 in the wild-type structure. Whereas the side-chain of A20 or R20 would be expected to clash with the preceding carbonyl oxygen (thus accounting for its frustrated energy landscape), the side-chain of D-Ala20 projects into solvent without perturbation of the Schellman motif. Calorimetric studies indicate that the increased stability of the D-Ala20 analogue (DeltaDeltaG(u) 1.5 kcal/mol) is entropic in origin, consistent with a conformational bias toward native-like conformations in the unfolded state. Studies of multiple substitutions at G20 and neighboring positions highlight the essential contributions of a glycine-specific tight turn and adjoining inter-subunit side-chain hydrogen bonds to the stability and architectural specificity of the intertwined dimer. Comparison of L- and D amino acid substitutions thus provides an example of the stereospecific control of an energy landscape by a helix-capping residue.

• Authors: Narendra Narayana, Nelson B Phillips, Qing-xin Hua, Wenhua Jia, Michael A Weiss
• Journal: Journal of molecular biology (J Mol Biol) Vol. 362 Issue 3 Pg. 414-29 (Sep 22 2006) ISSN: 0022-2836 [Print] Netherlands
• PMID: 16930618 (Publication Type: Journal Article, Research Support, N.I.H., Extramural, Research Support, Non-U.S. Gov't)

Chemical References

• HNF1A protein, human
• Hepatocyte Nuclear Factor 1-alpha

Topics

• Amino Acid Motifs
• Amino Acid Substitution
• Biophysical Phenomena
• Biophysics
• Crystallography, X-Ray
• Diabetes Mellitus, Type 2 (etiology, genetics, metabolism)
• Dimerization
• Hepatocyte Nuclear Factor 1-alpha (chemistry, genetics)
• Humans
• In Vitro Techniques
• Insulin-Secreting Cells (metabolism)
• Models, Molecular
• Point Mutation
• Protein Conformation
• Protein Folding
• Protein Structure, Quaternary
• Thermodynamics