Abstract
Plant defensins are best known for their antifungal activity and contribution to the plant immune system. The defining feature of plant defensins is their three-dimensional structure known as the cysteine stabilized alpha-beta motif. This protein fold is remarkably tolerant to sequence variation with only the eight cysteines that contribute to the stabilizing disulfide bonds absolutely conserved across the family. Mature defensins are typically 46–50 amino acids in length and are enriched in lysine and/or arginine residues. Examination of a database of approximately 1200 defensin sequences revealed a subset of defensin sequences that were extended in length and were enriched in histidine residues leading to their classification as histidine-rich defensins (HRDs). Using these initial HRD sequences as a query, a search of the available sequence databases identified over 750 HRDs in solanaceous plants and 20 in brassicas. Histidine residues are known to contribute to metal binding functions in proteins leading to the hypothesis that HRDs would have metal binding properties. A selection of the HRD sequences were recombinantly expressed and purified and their antifungal and metal binding activity was characterized. Of the four HRDs that were successfully expressed all displayed some level of metal binding and two of four had antifungal activity. Structural characterization of the other HRDs identified a novel pattern of disulfide linkages in one of the HRDs that is predicted to also occur in HRDs with similar cysteine spacing. Metal binding by HRDs represents a specialization of the plant defensin fold outside of antifungal activity.
Highlights
Plant defensins are a remarkable family of proteins
The first three hisitidine-rich defensins we identified were from N. benthamiana (NbD1, NbD2, and NbD3) (Figure S1A)
Histidine-rich plant defensins are a new subclass of plant defensins that in some cases have both
Summary
Plant defensins are a remarkable family of proteins. They are defined by a conserved three-dimensional structure consisting of three beta strands and a single alpha helix stabilized by four disulfide bonds forming a fold known as the cysteine-stabilized alpha-beta (CSαβ) motif [1]. A role for the histidine rich defensins in plant defence cannot be discounted as a cysteine/histidine rich DC-1 domain protein from Capsicum annum has a role in protecting plants against microbial pathogens [12] In another example, the peptides shepherin I and II from Shepherds purse, Capsella bursa-pastoris are enriched in glycine and histidine residues and are active against Gram-negative bacteria and fungi. The peptides shepherin I and II from Shepherds purse, Capsella bursa-pastoris are enriched in glycine and histidine residues and are active against Gram-negative bacteria and fungi These peptides of 28 and 38 amino acids respectively are derived from a larger precursor and have a random coil structure [13]. Two of the HRDs had antifungal activity, demonstrating an overlap between metal binding and antifungal functions of histidine rich plant defensins
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