Interactions of a glutamate-aspartate binding protein with the glutamate transport system of Escherichia coli

Abstract

Escherichia coli cells cultured with succinate as the carbon source display apparent K-m values for the uptake of L-glutamate of 10 muM in the absence of added sodium ion and 0.7 muM in the presence of an optimal level of sodium ion (15 to 50 mM). The glutamate transport system of the succinate cultured cells is noncompetitively inhibited by L-aspartate. A protein which binds glutamate and aspartate with K-D values of 0.7 and 1.2 muM, respectively, is released from the succinate cultured cells by osmotic shock or with the formation of spheroplasts during the preparation of membrane vesicles. The membrane vesicles of succinate cultured cells do not retain the whole cell capacity for L-glutamate uptake, but do retain much of the whole cell capacity for L-aspartate uptake. Culture of E. coli cells with glucose as carbon source causes a 2- to 3-fold repression of glutamate-aspartate binding protein but does not affect the velocity component of glutamate transport. As shown by other workers, the glutamate transport system of glucose cultured cells displays a sodium affected K-m value (FRANK, L., AND HOPKINS, I. (1969) J. Bacteriol. 100, 329-336) and is noncompetitively inhibited by L-aspartate (HALPERN, Y. S., AND EVEN-SHOSHAN, A. (1967) J. Bacteriol. 93, 1009-1016). Membrane vesicles prepared from glucose cultured cells retain the whole cell capacity for the uptake of glutamate (LOMBARDI, J. F., AND KABACK, H. R. (1972) J. Biol. Chem. 247, 7844-7857). The glutamate transport system of E. coli strain W appears to be conditionally dependent on the presence of the osmotic shock-releasable glutamate-aspartate binding protein. The results are interpreted to suggest that the binding protein-ligand complex acts as a substrate which is competitive with unbound substrate(s) for a sodium affected translocation process; the organization and specificity of which are dependent on the carbon source of the culture.