Crystal Structures of a T4-lysozyme Duplication-extension Mutant Demonstrate that the Highly Conserved β-Sheet Region has Low Intrinsic Folding Propensity

• Published in: Journal of molecular biology Vol. 316; no. 4; pp. 931 - 940
• Main Authors: Sagermann, Martin, Matthews, Brian W.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.03.2002
• Subjects: Amino Acid Sequence; Amino Acid Substitution; Bacteriophage T4 - enzymology; Bacteriophage T4 - genetics; Binding Sites; Chitinases - chemistry; Conserved Sequence; Crystallization; Crystallography, X-Ray; Evolution, Molecular; Gene Amplification; lysozyme; Models, Molecular; Molecular Sequence Data; Muramidase - chemistry; Muramidase - genetics; Muramidase - metabolism; Mutation - genetics; Phage T4; protein evolution; Protein Folding; Protein Structure, Secondary; Sequence Alignment; sequence duplication; sequence extension; β-sheet; β-sheet; lysozyme; protein evolution; sequence duplication; sequence extension
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1006/jmbi.2001.5376

Residues 24 to 35 of T4 lysozyme correspond to the second and third strands of a region of β-sheet that is highly conserved in all known lysozyme and chitinase structures. To evaluate the intrinsic propensity of these amino acid residues to form a defined structure they were added at the C terminus of the native protein, together with a dipeptide linker. Two crystal structures of this active, mutant protein were obtained, to 1.9 Å and 2.3 Å resolution, respectively. Even though the crystal conditions are similar, the appended sequence adopts very different secondary structures. In one case it is weakly structured and appears to extend through the active-site cleft, perhaps in part adding an extra strand to the original β-sheet. In the other crystal form the extension is largely α-helical. The formation of these alternative structures shows that the sequence does not have a strong intrinsic propensity to form a unique fold (either β-sheet or otherwise). The results also suggest that structural conservation during evolution does not necessarily depend on sequence conservation or the conservation of folding propensity.