Protein Science, Vol 2, Issue 9 1411-1428, Copyright © 1993 by Cold Spring Harbor Laboratory Press

ARTICLE

Use of proline mutants to help solve the NMR solution structure of type III antifreeze protein

H. CHAO, P. L. DAVIES, B. D. SYKES and F. D. SONNICHSEN
Department of Biochemistry, Queen's University, Kingston, Ontario K7L 3N6, Canada

To help understand the structure/function relationships in antifreeze proteins (AFP), and to define the motifs required for ice binding, a Type III AFP suitable for two-dimensional (2D) NMR studies was produced in Escherichia coli. A synthetic gene for one of the Type III AFP isoforms was assembled in a T7 polymerase-directed expression vector. The 67-amino acid-long gene product differed from the natural AFP by inclusion of an N-terminal methionine but was indistinguishable in activity. The NMR spectra of this AFP were complicated by cis-trans proline isomerization from the C-terminal sequence YPPA. Substitution of this sequence by YAA eliminated isomer signals without altering the activity or structure of the mutant AFP. This variant (rQAE m1.1) was selected for sequential assignment and the secondary structure determination using 2D (1)H NMR spectroscopy. Nine {beta}-strands are paired to form two triple-stranded antiparallel sheets and one double-stranded antiparallel sheet. Two further proline replacements, P29A and P33A, were made to delineate the role of conserved prolines in Type III AFP. These mutants were valuable in clarifying ambiguous NMR spectral assignments amongst the remaining six prolines of rQAE m1.1. In contrast to the replacement of the C-terminal prolyl residues, the exchange of P29 and P33 caused some structural changes and significantly decreased protein solubility and antifreeze activity.
CiteULike    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?