Lack of evidence for phosphorylation of Arabidopsis thaliana PII:: implications for plastid carbon and nitrogen signaling

The PII signal transduction protein is regulated by covalent modification in most prokaryotic organisms. In enteric bacteria PII is uridylylated on a specific tyrosine residue in the T-loop region, while in certain cyanobacteria it is phosphorylated at the serine residue two positions away from the equivalent modified tyrosine of enteric bacteria. Covalent modification functions primarily to signal cellular nitrogen status in prokaryotes. Here we have examined the phospho-status of Arabidopsis thaliana PII under various growth conditions employing a variety of techniques, including in vivo labeling, phosphospecific antibodies, protein phosphatase treatment, mass spectrometry and protein kinase assays. All results indicate that plant PII is not regulated by phosphorylation. Edman sequencing of immunoprecipitated A. thaliana PII revealed the N-terminal sequences AQISSD and QISSDY, indicating that the mature protein is cleaved from its transit peptide in vivo at the site(s) predicted by ChloroP. Western blot analysis also demonstrated that plant PII protein expression varies little with nutrient regime. (C) 2004 Elsevier B.V. All rights reserved.