VERY-LOW-DENSITY LIPOPROTEIN RECEPTOR FROM MAMMARY-GLAND AND MAMMARY EPITHELIAL-CELL LINES BINDS AND MEDIATES ENDOCYTOSIS OF M(R)-40,000 RECEPTOR-ASSOCIATED PROTEIN

We here report that the M(r) 40,000 receptor associated protein (RAP), previously found to bind to alpha(2),-macroglobulin receptor/low dens ity lipoprotein receptor related protein (alpha(2)MR/LRP) and glycopro tein 330 (gp330), binds to an M(r) 105,000 membrane protein from bovin e mammary gland, human mamma tumors and mammary epithelial cell lines. We have purified this protein from bovine and human sources. N-termin al amino acid sequencing and immunoblotting analyses showed that the p rotein was identical or closely related to very low density lipoprotei n receptor (VLDL-R). Experiments with the human mamma carcinoma cell l ine MCF-7 showed that this receptor was able to mediate an efficient e ndocytosis of RAP. These novel findings strongly suggest that RAP func tions as a modulator of ligand binding to VLDL-R, similarly to alpha(2 )MR/LRP and gp330.