G-proteins mediate signal transfer from receptors to effector systems. In their
guanosine 5'-triphosphate (
GTP)-bound form,
G-protein alpha-subunits activate effector systems. Termination of
G-protein activation is achieved by the high-affinity
GTPase [E.C. 3.6.1.-] of their alpha-subunits. Like
GTP,
inosine 5'-triphosphate (
ITP) and
xanthosine 5'-triphosphate (
XTP) can support effector system activation. We studied the interactions of
GTP,
ITP, and
XTP with the
retinal G-protein,
transducin (TD), and with
G-proteins in HL-60
leukemia cell membranes. TD hydrolyzed
nucleoside 5'-triphosphates (NTPs) in the order of efficacy
GTP >
ITP >
XTP. NTPs eluted TD from rod outer segment disk membranes in the same order of efficacy.
ITP and
XTP competitively inhibited TD-catalyzed
GTP hydrolysis. In HL-60 membranes, the
chemoattractants N-formyl-L-methionyl-L-leucyl-
L-phenylalanine (fMLP) and
leukotriene B4 (
LTB4) effectively activated
GTP and
ITP hydrolysis by Gi-
proteins. fMLP and
LTB4 were at least 10-fold more potent activators of
ITPase than of
GTPase.
Complement C5a effectively activated the
GTPase of Gi-
proteins but was only a weak stimulator of
ITPase. The potency of C5a to activate
GTP and
ITP hydrolysis was similar. The fMLP-stimulated
GTPase had a lower Km value than the fMLP-stimulated
ITPase, whereas the opposite was true for the Vmax values. fMLP, C5a, and
LTB4 did not stimulate
XTP hydrolysis. Collectively, our data show that
GTP,
ITP, and
XTP bind to
G-proteins with different affinities, that
G-proteins hydrolyze NTPs with different efficacies, and that
chemoattractants stimulate
GTP and
ITP hydrolysis by Gi-
proteins in a receptor-specific manner. On the basis of our results and the data in the literature, we put forward the hypothesis that
GTP,
ITP, and
XTP act as differential signal amplifiers and signal sorters at the
G-protein level.