Ras proteins as signal transmitters

The Ras proteins play a central role as signal
transmitters. They belong to the group of G proteins
(guanosine-residue-binding proteins with
signal-transmitting functions, see p. 266). The
binding of growth factor, e.g., PDGF, activates
the Ras protein by stimulating the conversion of
associated GDP (guanosine diphosphate) to GTP
(guanosine triphosphate) and triggering a short
time-limited signal that initiates cell division.
The signal is terminated by inactivation of Ras
by a GTPase-activating protein (GAP), which
converts GTP into GDP.Mutation of the Ras protein
or of GAP can remove the time limit of the
cell-stimulating signals and result in an active
condition with uncontrolled cell division. This
can lead to a tumor with uncontrolled growth
(malignancy). Several mutations have been defined
in the pertinent genes.