The complete amino acid sequence phospholipid-transfer protein isolated from spinach leaves has been determined. The primary structure of the spinach protein was elucidated by analyses by HPLC of cyanogen bromide fragments and peptides obtained by tryptic digestions. The single polypeptide chain of the spinach protein consists of 91 amino acid residues. The protein contains six cysteines whereas phenylalanine and glutamine are absent. The present data, which are the first to be obtained with a phospholipid-transfer protein from a photosynthetic tissue, are compared to the amino acid sequences determined with plant and animal proteins involved in the intracellular transport of hydrophobic compounds.Several proteins involved in the intracellular transport or binding of hydrophobic compounds: phospholipids, fatty acids, glycolipids, cholesterol, vitamin A, have been discovered (for reviews, see [l -111. In particular, proteins able to bind or transfer phospholipids, called phospholipid-transfer proteins, have been purified to homogeneity from animals [l -5, 10, 111, higher plants [3,4, 8, 121, yeasts [I31 or bacteria [14]. These proteins have in common the property of facilitating an intermembrane transfer of phospholipids. However, they differ in specificity. Some of these proteins are specific for phosphatidylcholine, like the protein specific to bovine liver [l, 2, 151, or for phosphatidylinositol and phosphatidylcholine, like the bovine brain protein [lo], whereas other proteins are non-specific. Non-specific proteins have been isolated from animal sources (bovine liver [2,4]) or plant tissues (spinach leaf [16], maize seed [17] or castor bean seed [12]); these proteins are for the major part basic (PI around 9 -10). A better knowledge of the structures of these proteins can provide information about their mode of action. The elucidation of the primary structure has only been obtained with two animal phospholipid-transfer proteins: the specific (18, 191 and the non-specific [20, 211 proteins from bovine liver. The recent obtention of high amounts of purified protein from spinach leaf [I61 has allowed a study of its structure; the NH,-terminal end has been determined in previous experiments [22]. This paper reports the complete amino acid sequence of this protein, which is the first phospholipidtransfer protein to be analyzed from a photosynthetic tissue. This structure was compared to that recently obtained with a similar protein purified from castor bean seeds [23]. MATERIALS AND METHODS Purification of phospholipid-transfer proteinPhospholipid transfer protein was isolated from spinach leaves as previously described [16]. The protein was desalted by gel filtration on a column of Sephadex G-25, equilibrated and eluted with 3.5 M acetic acid. Amino acid analysisAmino acid analyses were performed as in [16] on a Technicon TSMl amino acid analyzer. Oxidized protein samples (4 x 1 nmol) were hydrolyzed in 6 M HCl for 24, 48, 72 and 120 h. The values for serine and threonine were determined after 24 h hydrolysis...