Structure-function relationships in spruce budworm antifreeze protein revealed by isoform diversity

Area of Science:

• Biochemistry
• Molecular Biology
• Cryobiology

Background:

• Spruce budworm (Choristoneura fumiferana) larvae produce antifreeze proteins (AFPs) for overwintering survival.
• Insect AFPs exhibit significantly higher thermal hysteresis activity than fish AFPs.
• AFPs function by noncolligatively lowering hemolymph freezing point via ice crystal interaction.

Purpose of the Study:

• Identify key residues for ice interaction in spruce budworm AFPs.
• Investigate the molecular basis for the hyperactivity of insect AFPs.
• Characterize novel AFP isoforms and their structural/functional properties.

Main Methods:

• Isolation and sequencing of six new spruce budworm AFP cDNA isoforms.
• Amino acid sequence comparison and classification based on 3' UTR length.
• Molecular modeling using NMR structure of a short isoform.
• Recombinant expression and activity assays of modified AFP isoforms.

Main Results:

• Identified six new AFP isoforms differing up to 36% in amino acid sequence.
• New isoforms possess multiple 'Thr-X-Thr' ice-binding motifs; some encode larger 12-kDa proteins with insertions.
• Molecular modeling indicates insertions form beta-helix loops, aligning ice-binding motifs.
• Substitutions at the first threonine of the motif are tolerated, while the second threonine is conserved.

Conclusions:

• Specific substitutions at the first threonine of the 'Thr-X-Thr' motif do not affect AFP activity.
• The second threonine residue is critical for ice binding and the hyperactive antifreeze properties.
• Structural insights into AFP-ice interactions provide a basis for understanding insect cold hardiness.