Franz Watzinger, Thomas Lion
(Children Cancer Reserch Institute, St. Anna Children's Hospital, A-1090 Vienna, AUSTRIA)
Figure 1 Mechanism of RAS regulation
The activity of RAS proteins is regulated by a cycle of guanine nucleotide binding
and hydrolysis. In the active state p21 is bound to GTP, in the inactive to GDP.
GEF (Guanine Nucleotide Exchange Factor) promotes dissociation of GDP and acts as
a positive regulator; GAP (GTPase activating protein) promotes hydrolysis of GTP
and acts as
a negative regulator. Pi, inorganic phosphate.
Figure 2 Topological structure of p21 The polypeptide chain of RAS p21 consists of six b-strands and five a-helices.
Loop 1, alias phosphate-binding (P-) loop (residues 10 to 16), switch regions
I (30 to 37), including loop 2 with adjacent residues, and II (60 to 67), including
loop 4 and a-helix 2, represent the active center of the molecule and are involved
in the binding interaction between p21RAS and GTP. N stands for the amino terminal,
C for the carboxy terminal end.
(modified figure reprinted from Seminars in Cancer Biology vol 3, (4), F Wittinghofer,
Tree-dimentional structure of p21H-ras and its implications, p189-198, 1992,
by permission of the publisher Academic Press).
Activating point mutations have been localized in codons 12, 13, 59, 61, 63,
116, 117, 119, and 146 (Barbacid, 1990). All of these alterations occur at or
near the guanine nucleotide binding sites. The effects of point mutations are
either reduced GTPase activity (if amino acids 12, 13, 59, 61, 63 are involved),
so that oncogenic RAS mutants are locked in the active GTP-bound state, or decreased
nucleotide affinity, and hence, increased exchange of bound GDP for cytosolic
GTP (if amino acids 116, 117, 119 or 146 are affected). The inefficient deactivation
of the active GTP-bound RAS proteins is intensified by the inability of GAPs
to stimulate the conversion to the inactive, GDP-bound state. All point mutations
cause an accumulation of activated RAS-GTP complexes, leading to continuous
signal transduction by facilitating accumulation of constitutively active, GTP-bound
RAS protein, and thus contributing to a malignant cell phenotype.