α-Ketoglutarate Controls the Ability of the Escherichia coli PII Signal Transduction Protein To Regulate the Activities of NRII (NtrB) but Does Not Control the Binding of PII to NRII

PII signal transduction proteins are among the most widely distributed signaling proteins in nature; these proteins are direct sensors of alpha-ketoglutarate and adenylylate energy charge and control receptors that are signal transduction proteins, metabolic enzymes, or permeases involved in nitrogen metabolism. Prior studies showed that alpha-ketoglutarate regulated the ability of PII to control the activities of glutamine synthetase adenylyltransferase (ATase) but did not affect the ability of PII to bind to ATase. Here, we show that a similar pattern of alpha-ketoglutarate regulation was obtained with another PII receptor, the two-component system transmitter protein NRII (NtrB). Although alpha-ketoglutarate was required for the binding of Pit to NRII, PII bound to NRII equally well as the concentration of alpha-ketoglutarate was varied through its physiological range. Variation of the concentration of alpha-ketoglutarate through its physiological range provided dramatic regulation of the ability of PII to activate the phosphatase activity of NRII and controlled the ability of PII to inhibit the autophosphorylation of NRII. Thus, PII control of NRII activities could be dissected into distinct binding and regulation steps, and when present in its physiological concentration range, alpha-ketoglutarate apparently played a role in only the latter step.