The Primary Structure of the Phosphatidylcholine‐Exchange Protein from Bovine Liver

Abstract

The phosphatidylcholine exchange protein from bovine liver consists of a single polypeptide chain and has a blocked N terminus. The protein contains an estimated 244 amino acid residues in accordance with a determined molecular weight of 28 000. The protease from mouse submaxillaris gland cleaved the citraconylated and S-carboxymethylated derivative of the exchange protein at one specific site (Argl4-GluI5) close to the N terminus. Analysis of the two resulting peptides showed that N-acetyl-methionine was the N-terminal residue and gave the sequence of the first 41 residues.The modified protein was also fragmented with the protease from Staphylococcus aureus. The peptides isolated represented 88 of the protein; their sequences were determined by manual and automated Edman degradation. Alignment of a number of these peptides gave the complete sequence of the N-terminal half up to position 322.Phospholipid exchange proteins catalyze the transfer of phospholipids between membranes. A number of these proteins, different in molecular weight, isoelectric point and transfer specificity, have been isolated from bovine liver [l], heart [2,3] and brain [4,5], rat liver [6 -81, small intestinal mucosa [9] and hepatoma [lo], sheep lung [ l l ] and potato tuber [12]. To date, three general classes of exchange proteins can be distinguished, that is the phosphatidylcholinespecific exchange protein [1,6,9,11], the exchange proteins that display a great preference for phosphatidylinositol [4 -6,9] and the non-specific exchange proteins [8, lo]. These proteins range in molecular weight between 10000 and 35000 and in isoelectric point between pH 5 and 9.