GidA is an FAD-binding protein involved in development of Myxococcus xanthus

A gene encoding a homologue of the Escherichia coli GidA protein (glucose-i nhibited division protein A) lies immediately upstream of agiU, a gene enco ding a WD-repeat protein required for motility and development in Myxococcu s xanthus. The GidA protein of M. xanthus shares about 48% identity overall with the small (approximate to 450 amino acid) form of GidA from eubacteri a and about 24% identity overall with the large (approximate to 620 amino a cid) form of GidA from eubacteria and eukaryotes. Each of these proteins ha s a conserved dinucleotide-binding motif at the N-terminus. To determine if GidA binds dinucleotide, the M. xanthus gene was expressed with a His6 tag in E. coli cells. Purified rGidA is a yellow protein that absorbs maximall y at 374 and 450 nm, consistent with FAD or FMN. Thin-layer chromatography (TLC) showed that rGidA contains an FAD cofactor. Fractionation and immunoc ytochemical localization show that full length GidA protein is present in t he cytoplasm and transported to the periplasm of vegetative-grown M. xanthu s cells. In cells that have been starved for nutrients, GidA is found in th e cytoplasm. Although GidA lacks an obvious signal sequence, it contains a twin arginine transport (Tat) motif, which is conserved among proteins that bind cofactors in the cytoplasm and are transported to the periplasm as fo lded proteins. To determine if GidA, like AgIU, is involved in motility and development, the gidA gene was disrupted. The gidA(-) mutant has wild-type gliding motility and initially is able to form fruiting bodies like the wi ld type when starved for nutrients. However, after several generations, a s table derivative arises, gidA*, which is indistinguishable from the gidA(-) parent on vegetative medium, but is no longer able to form fruiting bodies . The gidA* mutant releases a heat-stable, protease-resistant, small molecu lar weight molecule that acts in trans to inhibit aggregation and gene expr ession of wild-type cells during development.