L. Goretzki et Bm. Mueller, Low-density-lipoprotein-receptor-related protein (LRP) interacts with a GTP-binding protein, BIOCHEM J, 336, 1998, pp. 381-386
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.