Selective interaction of AGS3 with G-proteins and the influence of AGS3 on the activation state of G-proteins.
AGS3 (activator of G-protein signaling 3) was isolated in a yeast-based functional screen for receptor-independent activators of heterotrimeric G-proteins. As an initial approach to define the role of AGS3 in mammalian signal processing, we defined the AGS3 subdomains involved in G-protein interaction, its selectivity for G-proteins, and its influence on the activation state of G-protein. Immunoblot analysis with AGS3 antisera indicated expression in rat brain, the neuronal-like cell lines PC12 and NG108-15, as well as the smooth muscle cell line DDT(1)-MF2. Immunofluorescence studies and confocal imaging indicated that AGS3 was predominantly cytoplasmic and enriched in microdomains of the cell. AGS3 coimmunoprecipitated with Galpha(i3) from cell and tissue lysates, indicating that a subpopulation of AGS3 and Galpha(i) exist as a complex in the cell. The coimmunoprecipitation of AGS3 and Galpha(i) was dependent upon the conformation of Galpha(i3) (GDP GTPgammaS (guanosine 5'-3-O-(thio)triphosphate)). The regions of AGS3 that bound Galpha(i) were localized to four amino acid repeats (G-protein regulatory motif (GPR)) in the carboxyl terminus (Pro(463)-Ser(650)), each of which were capable of binding Galpha(i). AGS3-GPR domains selectively interacted with Galpha(i) in tissue and cell lysates and with purified Galpha(i)/Galpha(t). Subsequent experiments with purified Galpha(i2) and Galpha(i3) indicated that the carboxyl-terminal region containing the four GPR motifs actually bound more than one Galpha(i) subunit at the same time. The AGS3-GPR domains effectively competed with Gbetagamma for binding to Galpha(t(GDP)) and blocked GTPgammaS binding to Galpha(i1). AGS3 and related proteins provide unexpected mechanisms for coordination of G-protein signaling pathways.
Bernard
M L
ML
Department of Pharmacology, Medical University of South Carolina, Charleston, South Carolina 29403, USA.
Peterson
Y K
YK
Chung
P
P
Jourdan
J
J
Lanier
S M
SM
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000378:metabolism; D002352:Carrier Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D004331:Drosophila melanogaster; D019206:GTP-Binding Protein alpha Subunits, Gi-Go / Q000378:metabolism*; D020962:Heterotrimeric GTP-Binding Proteins / Q000378:metabolism*; D006801:Humans; D008969:Molecular Sequence Data; D009130:Muscle, Smooth / Q000378:metabolism; D009474:Neurons / Q000378:metabolism; D016716:PC12 Cells; D051381:Rats; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
D002352:Carrier Proteins; C498913:Gpsm1 protein, rat; D011994:Recombinant Proteins; D019206:GTP-Binding Protein alpha Subunits, Gi-Go; D020962:Heterotrimeric GTP-Binding Proteins