Heterotrimerization of P-II-like signalling proteins: implications for P-II-mediated signal transduction systems

P-II-like signalling molecules are trimeric proteins composed of 12-13 kDa polypeptides encoded by the glnB gene family. Heterologous expression of a cyanobacterial glnB gene in Escherichia coli leads to an inactivation of E. coli's own P-II signalling system. In the present work, we show that this effect is caused by the formation of functionally inactive heterotrimers be tween the cyanobacterial glnB gene product and the E. coli P-II paralogues GlnB and GlnK. This led to the discovery that GlnK and GlnB of E. coli also form heterotrimers with each other. The influence of the oligomerization p artner on the function of the single subunit was studied using heterotrimer ization with the Synechococcus P-II protein. Uridylylation of GlnB and GlnK was less efficient but still possible within these heterotrimers. In contr ast, the ability of GlnB-UMP to stimulate the adenylyl-removing activity of GlnE (glutamine synthetase adenylyltransferase/removase) was almost comple tely abolished, confirming that rapid deadenylylation of glutamine syntheta se upon nitrogen stepdown requires functional homotrimeric GlnB protein. Re markably, however, rapid adenylylation of glutamine synthetase upon exposin g nitrogen-starved cells to ammonium was shown to occur in the absence of a functional GlnB/GlnK signalling system as efficiently as in its presence.