The X-ray crystal structure and biochemical analysis of a native basic pathogenesis-related-1 protein from Mucuna sempervirens (Fabaceae) floral nectar

BACKGROUND AND AIMS: Pathogenesis-Related 1 (PR-1) proteins are small secreted proteins that typically accumulate upon pathogen attack and serve as hallmarks of plant immune activation. Despite their widespread use as defence markers, their biochemical properties and native structural features remain largely unresolved. This study aimed to isolate, characterize and determine the crystal structure of a native PR-1 protein (MsPR-1b) that predominates in the floral nectar of Mucuna sempervirens, a woody leguminous vine, to provide insight into the molecular diversity and possible functional specialization of PR-1 proteins. METHODS: MsPR-1b was purified from raw nectar using ion-exchange and size-exclusion chromatography. The purified protein was analysed for molecular mass and glycosylation status, and its three-dimensional structure was determined by X-ray crystallography at 2.0 Å resolution. The full-length complementary DNA sequence was obtained through 3' and 5' rapid amplification of complementary DNA ends PCR, and tissue-specific expression was examined by quantitative PCR. Lipid-binding ability and antimicrobial activity were assessed by fluorescence spectroscopy and the disc diffusion method, respectively. KEY RESULTS: The MsPR-1b gene encodes a 164-amino-acid precursor containing an N-terminal signal peptide. Expression was detected predominantly in the nectary. The mature MsPR-1b is a small basic secreted protein (15 134 Da, pI 9.4) that lacks glycosylation. The crystal structure, representing the first native PR-1 protein purified from its natural source, revealed a homodimer. Each monomer adopts the conserved α-β-α sandwich fold characteristic of the CAP superfamily and contains two intrachain disulphide bonds. MsPR-1b displays no detectable lipid-binding capacity or direct antimicrobial activity. CONCLUSIONS: MsPR-1b provides the first structural view of a native plant PR-1 protein and indicates that nectar-secreted PR-1s might perform specialized, non-defensive functions distinct from canonical pathogen-related roles.