Proteolytic processing of the 600 kd low density lipoprotein receptor-related protein (LRP) occurs in a trans-Golgi compartment

The low density lipoprotein receptor-related protein (LRP) is a cell surface glycoprotein that binds and transports plasma lipoproteins enriched in apolipoprotein E. It is synthesized in the endoplasmic reticulum as a transmembrane glycosylated precursor that migrates with an apparent molecular mass of about 600 kd on SDS-polyacrylamide gels. After it reaches the Golgi complex, the protein is cleaved to generate two subunits with apparent molecular masses of ∼515 and 85 kd respectively. The larger NH₂-terminaI α-subunit lacks a membrane-spanning region. It remains attached to the membrane through noncovalent association with the smaller COOH-terminal β-subunit. Proteolysis occurs at the sequence RHRR, which resembles the sequence RKRR at the proteolytic site in the receptors for insulin and insulin-like growth factor-1 (IGF-1), the only other cell surface receptors known to undergo proteolytic processing. Proteolysis of LRP occurs coincident with the conversion of the N-linked carbohydrates to the mature endoglycosidase H-resistant, neuraminidase-sensitive form. Proteolysis is prevented by brefeldin A, which blocks transport to the Golgi complex. These data raise the possibility that LRP and the receptors for insulin and IGF-1 are processed by a specific endoprotease that recognizes protein with extended basic sequences and resides in the frans-Golgi complex or in post-Golgi vesicles of the constitutive secretory pathway.