PII-like signal transduction proteins are found in all three domains of life and have been shown to play key roles in the control of bacterial nitrogen assimilation. This communication reports the first target protein of an archaeal PII-like protein, representing a novel PII receptor. The GlnK1 protein of the methanogenic archaeon Methanosarcina mazei strain Gö1 interacts and forms stable complexes with glutamine synthetase (GlnA1). Complex formation with GlnK1 in the absence of metabolites inhibits the activity of GlnA1. On the other hand, the activity of this enzyme is directly stimulated by the effector molecule 2-oxoglutarate. Moreover, 2-oxoglutarate antagonized the inhibitory effects of GlnK1 on GlnA1 activity but did not prevent GlnK1/GlnA1 complex formation. On the basis of these findings, we hypothesize that besides the dominant effector molecule 2-oxoglutarate, the nitrogen sensor GlnK1 allows finetuning control of the glutamine synthetase activity under changing nitrogen availabilities and propose the following model. (i) Under nitrogen limitation, increasing concentrations of 2-oxoglutarate stimulate maximal GlnA1 activity and transform GlnA1 into an activated conformation, which prevents inhibition by GlnK1. (ii) Upon a shift to nitrogen sufficiency after a period of nitrogen limitation, GlnA1 activity is reduced by decreasing internal 2-oxoglutarate concentrations through diminished direct activation and by GlnK1 inhibition.