Novel structures of PII signal transduction proteins from oxygenic phototropic organisms

Abstract

PII proteins constitute one of the most widely distributed families of signal transduction proteins, whose representatives are present in archaea, bacteria and plants. They play a pivotal role to control the nitrogen, carbon and energy status of the cell in response to the central metabolites ATP, ADP and 2-oxoglutarate (2-OG). These signals from central metabolites are integrated by PII proteins and transmitted to the regulatory targets (protein modifying enzymes, metabolic enzymes, transporters and transcription factors). In oxygenic phototrophic organisms, from cyanobacteria to higher plants, the controlling enzyme of arginine synthesis, N-acetyl-L-glutamate kinase (NAGK) is a major PII target, whose activity responds to the cellular metabolites via PII signalling. In this work, novel crystal structures of PII signal transduction proteins from oxygenic phototrophs (Synechococcus elongatus and Chlamydomonas reinhardtii) in the presence of signalling metabolites and in complex with NAGK are reported. These structures give deeper insights into PII-mediated mechanism and regulation which are in accordance with the obtained biochemical data. The novel role of glutamine as a signalling molecule in C. reinhardtii is elucidated for the first time, which highlights the nitrogen regulation at a different level. Further, the interpretation of these structures together with the comparison of aminoacid sequences sheds light on the evolutionary adaptation of PII signal transduction from cyanobacteria to plastids.