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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 approximately 515 and 85 kd respectively. The larger NH2‐terminal alpha‐subunit lacks a membrane‐spanning region. It remains attached to the membrane through noncovalent association with the smaller COOH‐terminal beta‐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 trans‐Golgi complex or in post‐Golgi vesicles of the constitutive secretory pathway.