Sensory properties of the P II signalling protein family

P II signalling proteins constitute one of the largest families of signalling proteins in nature. An even larger superfamily of trimeric sensory proteins with the same architectural principle as P II proteins appears in protein structure databases. Large surface‐exposed flexible loops protrude from the intersubunit faces, where effector molecules are bound that tune the conformation of the loops. Via this mechanism, P II proteins control target proteins in response to cellular ATP / ADP levels and the 2‐oxoglutarate status, thereby coordinating the cellular carbon/nitrogen balance. The antagonistic ( ATP versus ADP ) and synergistic (2‐oxoglutarate and ATP ) mode of effector molecule binding is further affected by P II ‐receptor interaction, leading to a highly sophisticated signalling network organized by P II . Altogether, it appears that P II is a multitasking information processor that, depending on its interaction environment, differentially transmits information on the energy status and the cellular 2‐oxoglutarate level. In addition to the basic mode of P II function, several bacterial P II proteins may transmit a signal of the cellular glutamine status via covalent modification. Remarkably, during the evolution of plant chloroplasts, glutamine signalling by P II proteins was re‐established by acquisition of a short sequence extension at the C‐terminus. This plant‐specific C‐terminus makes the interaction of plant P II proteins with one of its targets, the arginine biosynthetic enzyme N ‐acetyl‐glutamate kinase, glutamine‐dependent.