Defensins belong to a diverse group of cationic antimicrobial peptides (CAPs) expressed in most plant and animal species as a first line of defense against invading microbes. The defensins generally dimerize and selectively bind phospholipid headgroups such as phosphatidylinositol 4,5-bisphosphate (PIP2) and phosphatidic acid (PA) using a a cationic grip configuration. Many of these defensins have been shown to permeabilize cell membranes, and once inside form oligomeric structures that rupture the membrane by a yet to be determined mechanism.
Our lab has previously determined the oligomeric structure of Nicotiana alata defensin 1 (NaD1) in complex with PIP21 and Nicotiana suaveolens defensin 7 in complex with PA2. These two structures have demonstrated the defensins ability to form vastly different oligomeric structures upon binding specific lipids, but neither structure has fully described the membrane attack mechanism.
In a continuous effort to unravel the crucial underlying permeabilization mechanism of defensins I determined the structure of NaD1 in complex with PA to 2.6Å. The complex crystallized as a 20-meric unit comprising of 10 NaD1-dimers and 14 PA bound in a symmetric pattern. The observed oligomer is flat, forming a defensin carpet, which is a striking difference compared to previous structures that displayed long coil-like topologies. The overall topology of the NaD1:PA oligomer reveals what we hypothesize to be a putative membrane disruption configuration that ultimately destabilizes cell membranes through direct lipid sequestering by carpeting the target cell plasma membrane.