Tricarboxylate-binding proteins of Salmonella typhimurium. Purification, crystallization, and physical properties

Citrate transport in Salmonella typhimurium involves inducible periplasmic components. Two forms of a tricarboxylate-binding protein, C1 and C2, were isolated, in high yield, from the periplasm of a cyclic AMP phosphodiesterase mutant. These immunologically cross-reactive Mr = 29,000 proteins were crystallized using ammonium sulfate. CD measurements indicated considerable secondary structure: 24% a helix, and 12% beta structure. The amino acid compositions of C1 and C2 were identical. The NH2-terminal sequence of C1 was determined; C2 was found to have a blocked NH2 terminus (pyroglutamate). C1 and C2 are products of the same gene (Somers, J. M., and Kay, W. W. (1983) Mol. Gen. Genet. 190, 20-26). C1 and C2 bound a variety of citrate analogues and organic acids, with a predominant specificity for tricarboxylates (citrate KD 1.4 X 10(-7) M), and both required a deprotonated central carboxyl group for binding. Citrate was not bound to C protein as either a salt or metal ion complex.