A Binding Protein Involved in the Transport of Cystine and Diaminopimelic Acid in Escherichia coZi*

Escherichin coli strain W is capable of actively transporting L-cystine as well as the structurally related compound OL,Ediaminopimelic acid. Cystine transport is mediated by two systems distinguishable on the basis of specificity and affinity for substrate. The cystine general system (K, = 3 x 10e7 M) also transports diaminopimelic acid and is inhibited by a variety of analogues, while the cystine specific system (K, = 2 X 1O-8 M) is much more selective. A mutant strain, D2W, has only the lower affinity general system (K, = 1 x 1OF M). The two systems display differing sensitivities toward osmotic shock. When cells are grown in a minimal medium, the general system is entirely lost from each strain by shock, whereas the specific system of W is unaffected. The shock fluids from these cells contain a single protein capable of binding cystine (the cystine general binding protein), and the binding is completely inhibited by diaminopimelate. This binding activity is reduced by 90% in mutants of strain D2W lacking the general transport system. When strain W is grown in an enriched medium, the general system is absent, but the activity of the specific system is increased 3-fold and can now be reduced by osmotic shock. The cystine general binding protein is absent from the shock fluid, but we now find the appearance of a cystine binding activity with a specificity similar to that of the specific transport system. The cystine general binding protein has been purified to homogeneity and has properties similar to those reported for other shock-releasable binding proteins. Its specificity closely parallels that of the general transport system. The results support the concept that the cystine general binding protein is involved in the recognition step of the general transport system.