Zusammenfassung
Feedback inhibition is a fundamental principle in signal transduction
allowing rapid adaptation to different stimuli. In mammalian cells,
the major feedback inhibitor for G-protein-coupled receptors (GPCR)
is G-protein-coupled receptor kinase 2 (GRK-2), which phosphorylates
activated receptors, uncouples them from G proteins and initiates
their internalization. The functions of GRK-2 are indispensable and
need to be tightly controlled. Dysregulation promotes disorders such
as hypertension or heart failure. In our search for a control mechanism
for this vital kinase, here we show that the Raf kinase inhibitor
protein (RKIP) is a physiological inhibitor of GRK-2. After stimulation
of GPCR, RKIP dissociates from its known target, Raf-1 (refs 6-8),
to associate with GRK-2 and block its activity. This switch is triggered
by protein kinase C (PKC)-dependent phosphorylation of the RKIP on
serine 153. The data delineate a new principle in signal transduction:
by activating PKC, the incoming receptor signal is enhanced both
by removing an inhibitor from Raf-1 and by blocking receptor internalization.
A physiological role for this mechanism is shown in cardiomyocytes
in which the downregulation of RKIP restrains beta-adrenergic signalling
and contractile activity.
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