Functional study of g protein signaling during growth and development of the social amoeba Dictyostelium discoideum
The social amoeba Dictyostelium discoideum senses the bacterial metabolite folic acid to track down bacteria. Previous studies suggested that the folic acid receptor is a serpentine receptor. In this study, I examined the transcriptional level changes of 46 putative serpentine receptor genes after folic acid stimulation. Seven candidate genes showed significantly up-regulated transcriptional levels, including fslA, fslB, fslH, fslJ, fslK, grlB, and grlD. Unfortunately, subsequent disruption of these genes did not affect folic acid sensing. Among these putative serpentine receptor gene mutants, grlB- cells exhibited delayed aggregation. Further studies suggest that GrlB is required for GABA binding. Detailed characterization of GABA metabolism indicates that GABA is used as an ancient signal during early development and culmination. Loss of GABA leads to a delay in early development and reduced detergent-resistant spores. GABA metabolism and signaling are regulated by distinct genes in different stages. In vegetative and early developmental stages, GABA is generated by the glutamate decarboxylase GadB and signals through GrlB. In late developmental stage, GABA is synthesized by the glutamate decarboxylase GadA and signals through GrlE. Mitochondrial GabT is required for GABA degradation, and DdvGAT is likely responsible for GABA secretion in culmination. Study of G proteins suggests that G protein alpha subunit Gα8 is probably involved in GABA signaling. Characterization of Gα8 also reveals that Gα8 promotes adhesion in the social amoeba. Here I propose a non-canonical model of how Gα8 works. Starvation triggers the up-regulation of both Gα8 and Gβγ. Gα8 appears to function as a heterotrimer. This heterotrimer interacts with unknown downstream effectors, which eventually promotes the expression of plasma membrane-anchored adhesion molecules or facilitates the membrane presenting of these adhesion molecules. Once the G protein heterotrimer is activated, Gα8 separates from Gβγ and no longer activates adhesion molecules. This activated Gα8 can mediate other signaling pathways, most notably the AprA or GABA signaling pathway.