Spatial Characteristics of the Aspartate Membrane Carrier and Its Relationship to the Glutamate Transport System in Membrane Vesicles of Escherichia coli K‐12 Cavalli

Abstract

Membrane vesicles of E. coli K-12 CS7, a strain genetically derepressed for glutamate permease, maintain low aspartate transport activity, like that of preparations of the wild-type parent. Growth of the parent CS101 on aspartate as the C or N source results in derepression of aspartate and glutamate transport. Growth of strain CS7 on aspartate derepresses aspartate transport to the same extent as in strain CS101, but only slightly increases the derepressed level of glutamate transport activity. The affinity of the membrane transport system for glutamate is enhanced by Na, while that for aspartate is not. Although the affinities for glutamate (23 .mu.M) and aspartate (12 .mu.M) are similar, aspartate does not inhibit glutamate transport, while glutamate competitively inhibits aspartate transport. Aspartate transport, but not glutamate transport, is competitively inhibited by C4 dicarboxylic acids, but 2-oxoglutarate competitively inhibits glutamate transport, but not aspartate transport. Competitive inhibition of L-aspartate transport by L-glutamate and by the 5-methyl ester of L-glutamate is abolished in the presence of 2-oxoglutarate, but 2-oxoglutarate does not affect the competitive inhibition of L-aspartate transport by D-aspartate and by DL-threo-3-hydroxyaspartate. The relationship between the 2 dicarboxylic amino acid transport systems and the spatial characteristics of the aspartate carrier are discussed.