Biochemical properties of phosphatidylcholine-binding proteins that share common antigenic determinants with Fc gamma...

Biochemical and immunological properties of biosynthetically radiolabeled phosphatidylcholine-(PC-) binding proteins were investigated. The PC-binding proteins were extracted from the detergent lysate of biosynthetically radiolabeled P388D1 cells by affinity chromatography on PC-Sepharose and filtered through a Sephadex G-100 gel column in the presence of 6 M urea. Isoelectric focusing of the gel-filtered materials in the presence of 6 M urea revealed the presence of a major protein component of pIe of 5.8 and minor heterogeneous cellular proteins. The yield of the electrofocused PC-binding proteins based on protein determination by Lowry's method ranged from 0.7 to 4 mg per 10(9) cells. The purified PC-binding proteins appeared to be tightly associated with Triton X-100 and phospholipids in the weight ratio of 0.57 and 0.05 g/g of proteins, respectively. The majority of lipids that could be extracted from the PC-binding proteins by chloroform/methanol (2:1 v/v) are free fatty acids, whereas lipids extracted from Pronase-treated PC-binding proteins contained phosphatidylethanolamine. By amino acid analysis, the purified PC-binding proteins were found to consist of a minimum of 417 amino acid residues, suggesting a minimum molecular weight of about 38 000 for this protein. Results of radiolabeling experiments with [3H]glucosamine and amino acid analysis both showed the presence of a mole of glucosamine per a mole of the PC-binding proteins, suggesting their glycoprotein nature. About 40% of the purified PC-binding proteins coprecipitated with monoclonal anti-Fc gamma 2bR antibody (2.4G2) in detergent-containing buffer, whereas only 6% of the isolated IgG binding proteins reacted with this antibody.(ABSTRACT TRUNCATED AT 250 WORDS)