TIGHT ATTACHMENT OF FATTY-ACIDS TO PROTEINS ASSOCIATED WITH MILK LIPID GLOBULE-MEMBRANE

Abstract

The proteinaceous coat associated with cytoplasmic side of milk lipid globule membranes (MLGM) was prepared from bovine and caprine milk by removal of membrane material with non-ionic detergent. These coat preparations, which were enriched in 2 major proteins, a glycoprotein of polypeptide MW, 67,000 (butyrophilin) and a non-glycosylated protein of polypeptide MW 155,000 (xanthine oxidase), contained small amounts of fatty acids which could not be removed by exhaustive extractions with organic solvents. Both butyrophilin and xanthine oxidase of obvine MLGM were excised and eluted from SDS[sodium dodecyl sulfate]-polyacrylamide-polyacrylamide gels and were shown to contain 1 to 2 moles of bound fatty acids per mol of protein. Palmitic, stearic and oleic acids were the predominant protein-bound fatty acids, but no specificity for binding of individual fatty acids was observed. The fatty acids were not rendered soluble in organic solvents when the protein preparations were incubated with phospholipases A or C or with trypsin. Treatment with 0.25 M NaOH at 100.degree. C for 1 h or with 1 M hydroxylamine at 4.degree. C for 16 h, however, released virtually all of the fatty acids associated with these proteins. Similar results were obtained with 2 major proteins, bands 3 and 4.1, of rat erythrocyte plasma membrane. By contrast, skeletal muscle actin and serum albumin had no bound fatty acids that could be released by alkali treatment. These results show that fatty acids are bound to a number of membrane-associated proteins, both glycosylated and unglycosylated, via linkages that resist purification of the proteins on SDS-polyacrylamide gel electrophoresis and are suggestive of covalent attachement of fatty acids to these proteins. The possible involvement of this acylation in processes characterized by local changes of membrane shape and plasticity is discussed.