Low-density-lipoprotein-receptor-related protein (LRP) interacts with a GTP-binding protein

The low-density-lipoprotein-receptor-related protein (LRP) binds and intern alizes numerous ligands, including lipoproteins, proteinase-inhibitor compl exes and others. We have shown previously that LRP-mediated ligand internal ization is dependent on cAMP-dependent protein kinase (PKA) activity. Here, we investigated whether ligation of LRP increases the intracellular cAMP l evel and PKA activity via a stimulatory GTP-binding protein, Treatment of L RP-expressing cell lines with the LRP ligands lactoferrin or urokinase-type plasminogen activator caused a significant elevation in cAMP and stimulate d PKA activity in a dose-dependent manner. Addition of the 39 kDa receptor- associated protein (RAP), an antagonist for ligand interactions with LRP, b locked the lactoferrin-induced increase in PKA activity, demonstrating a re quirement for ligand binding to LRP. Incubation of cell membrane fractions with lactoferrin increased GTPase activity in a time- and dose-dependent ma nner, and treatment with LRP ligands suppressed cholera-toxin-mediated ADP- ribosylation of the G(s)alpha subunit of a heterotrimeric G-protein. Affini ty precipitation of LRP with RAP resulted in co-precipitation of two isofor ms of G(s)alpha from detergent extracts. We thus conclude that LRP is a sig nalling receptor that associates directly with a stimulatory heterotrimeric G-protein and activates a downstream PKA-dependent pathway.