Ultra-high resolution crystal structure of a dimeric defensin SPE10.

Defensins are key players of the innate immune system known to act against bacteria, fungi and viruses. Here we report the 0.98-Å crystal structure of SPE10, a dimeric plant defensin. SPE10 associates as a dimer through a unique amino acid triplet involving residues R36-W42-R40. The helix from one subunit interacts with arginines R36 and R40 from the other subunit, forming a sheet-like dimer with a highly extended molecular surface. A conserved hydrophobic patch on the molecular head largely overlaps with the putative receptor-binding site previously reported for another defensin. Structural analysis and mutational studies indicate that the dimeric association of SPE10 is relevant to its function, and that the hydrophobic patch on the molecular head is required for its antifungal activity.