SELECTION OF G-BETA SUBUNITS WITH POINT MUTATIONS THAT FAIL TO ACTIVATE SPECIFIC SIGNALING PATHWAYS IN-VIVO - DISSECTING CELLULAR-RESPONSESMEDIATED BY A HETEROTRIMERIC G-PROTEIN IN DICTYOSTELIUM-DISCOIDEUM
T. Jin et al., SELECTION OF G-BETA SUBUNITS WITH POINT MUTATIONS THAT FAIL TO ACTIVATE SPECIFIC SIGNALING PATHWAYS IN-VIVO - DISSECTING CELLULAR-RESPONSESMEDIATED BY A HETEROTRIMERIC G-PROTEIN IN DICTYOSTELIUM-DISCOIDEUM, Molecular biology of the cell, 9(10), 1998, pp. 2949-2961
In Dictyostelium discoideum, a unique G beta subunit is required for a
G protein-coupled receptor system that mediates a variety of cellular
responses. Binding of cAMP to cAR1, the receptor linked to the G prot
ein G2, triggers a cascade of responses, including activation of adeny
lyl cyclase, gene induction, actin polymerization, and chemotaxis. Nul
l mutations of the cAR1, G alpha 2, and G beta genes completely impair
all these responses. To dissect specificity in G beta gamma signaling
to downstream effectors in living cells, we screened a randomly mutag
enized library of G beta genes and isolated G beta alleles that lacked
the capacity to activate some effecters but retained the ability to r
egulate others. These mutant G beta subunits were able to link cAR1 to
G2, to support gene expression, and to mediate cAMP-induced actin pol
ymerization, and some were able to mediate to chemotaxis toward cAMP.
None was able to activate adenylyl cyclase, and some did not support c
hemotaxis. Thus, we separated in vivo functions of G beta gamma by mak
ing point mutations on G beta. Using the structure of the heterotrimer
ic G protein displayed in the computer program CHAIN, we examined the
positions and the molecular interactions of the amino acids substitute
d in each of the mutant G beta s and analyzed the possible effects of
each replacement. We identified several residues that are crucial for
activation of the adenylyl cyclase. These residues formed an area that
overlaps but is not identical to regions where bovine Gt beta gamma i
nteracts with its regulators, G alpha and phosducin.