A phosphatidylethanolamine-containing complex on human B cells that mediates rosette formation with mouse erythrocytes.

The maturation-associated human B cell rosette receptor (MER) for mouse erythrocytes has been solubilized from B cells by mild trypsinization. It specifically agglutinates mouse red cells. Material with hemagglutinating activity partitioned into the lipid-soluble phase of a Folch partition of the trypsin extract was sensitive to phospholipase C and alkali, and on two-dimensional thin layer chromatography, it co-migrated principally with phosphatidylethanolamine (PE). Phosphatidylcholine, the major lipid present, was inactive. The relationship of phospholipid structure to hemagglutinating activity has been described. PE in the crude trypsin extract was associated with unidentified glycoprotein and albumin. Material containing hemagglutinating lipid bound to a wheat germ lectin-Sepharose column and was released by N-acetylglucosamine, indicating that the PE was complexed with glycoprotein. When the crude trypsin extract or eluate from the lectin column was extracted with aqueous phenol, hemagglutinin in the aqueous phase no longer bound to wheat germ lectin-Sepharose; however, albumin was greatly enriched, indicating that some of the PE exists in a complex with albumin. The molar ratio of PE to albumin was approximately 200:1. After delipidation, this albumin (in molar excess) inhibited hemagglutination by PE in the same way as a recently described subclass of serum albumin. Studies with phospholipase-treated B cells were also consistent with PE being the MER. We conclude that MER is PE, existing in a complex containing glycoprotein and a subclass of albumin. The capacity to form rosettes can be transferred to nonrosetting Raji B cells by the complex, but not pure PE, indicating that the proteins may be involved in orienting PE correctly for it to function as the MER.