Four 9-kDa proteins excreted by somatic embryos of grapevine are isoforms of lipid-transfer proteins

Four 9-kDa small extracellular proteins produced by embryogenic cultures in the absence of auxin have been purified from the extracellular medium of grapevine somatic embryo cultures through cation-exchange chromatography and hydrophobic-interaction chromatography. The partial amino-acid sequences reflect high similarities between the four proteins as well as with the sequences established for carrot, spinach, millet and maize nonspecific lipid-transfer proteins. All these sequences show conservation of three cysteines at positions 4, 14 and 30-32, as well as glycine, valine, tyrosine and lysine residues at positions 5, 7, 17 and 37, respectively. In-vitro lipid-transfer assays reveal that the four proteins catalyze the transfer of phosphatidylcholine from liposomes towards mitochondria with an efficiency similar or higher than that of a purified maize lipid-transfer protein.