Sensory properties of the PII 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.