Biochemical Characterization of a Lipid-Transfer Protein in Squash Nectar
Many plants have evolved to attract pollinators by offering a reward of floral nectar. Nectar is a sugar-rich liquid secreted from the floral nectaries of plants. Nectar is primarily composed of sugar and various nutrients, but it also has several proteins at high concentrations. A class of proteins commonly found in nectar are lipid-transfer proteins (LTPs). This study was performed to characterize a lipid-transfer protein (CpLTP) secreted into Cucurbita pepo (squash) nectar. LTPs are small proteins ~70-90 amino acids long that are rich in cysteine. The intramolecular disulfide bonds formed by the cysteine residues create a hydrophobic pocket that has been shown to bind phospholipids. To characterize this protein, CpLTP was expressed in and purified from Escherichia coli SHuffle cells. A fluorescence assay showed that CpLTP preferentially bound shorter saturated fatty acids over longer ones (e.g., 14:0 vs. 18:0), and unsaturated fatty acids were always preferred to saturated ones (e.g., 18:1 vs. 18:0). The purified protein also displayed antimicrobial activity toward certain filamentous fungi. These results support a role for CpLTP in plant defense, particularly in preventing microbial growth in nectar.