Abstract
Receptors of the seven transmembrane domain family are coupled to
heterotrimeric G proteins 1. Binding of ligand to these receptors
induces dissociation of the heterotrimeric complex into free GTP-Galpha
and Gbetagamma subunits, which then interact with their respective
effector molecules to stimulate specific cellular responses. In some
cases, these cellular responses involve mitogenic signalling 2.
The mitogen-activated protein (MAP) kinase cascade is initiated by
the protein kinase cRaf1 and links growth factor receptor signalling
to cell growth and differentiation 3. The main activator of cRaf1
is the small GTP-binding protein Ras 4, and the binding of cRaf1
to GTP-Ras translocates cRaf1 to the plasma membrane, where it is
activated 5. It has been reported that cRaf1 associates directly
with the beta subunit of heterotrimeric G proteins in vitro, and
with the betagamma subunit complex in vivo 6, but the role of this
association is not yet understood. Here, we show that cRaf1 associates
with Gbeta1gamma2, and that this association in mammalian cells is
significantly enhanced when active p21(Ras) is present or when cRaf1
is otherwise targeted to the membrane. Association with Gbeta1gamma2
has no effect on the kinase activity of cRaf1, but cRaf1 can affect
Gbetagamma-mediated signalling events. Thus, membrane-localised cRaf1
inhibits G-protein-coupled receptor (GPCR)-stimulated activation
of phospholipase Cbeta (PLCbeta) by sequestration of Gbetagamma subunits,
an effect also observed with endogenous levels of cRaf1. Our data
suggest that cRaf1 may be an important regulator of signalling by
Gbetagamma, particularly in those GPCR systems that stimulate the
MAP kinase cascade through the activation of p21(Ras).
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