Acquisition of hypoxia inducibility by oxygen sensing N-terminal cysteine oxidase in spermatophytes
N-terminal cysteine oxidases (NCOs) are enzymes that use molecular oxygen to oxidize the amino-terminal cysteine of specific proteins, thereby initiating the proteolytic N-degron pathway and thus conferring them oxygen-dependent instability. To expand the characterization of the plant family of NCOs (PCOs), we performed a phylogenetic analysis across different plant taxa in terms of sequence similarity and transcriptional regulation. Based on this survey, we propose a distinction of PCOs into two main groups: A-type and B-type sequences. A-type PCOs are conserved across all plant species and are generally unaffected at the mRNA level by oxygen availability. Instead, B-type PCOs differentiated in spermatophytes to acquire specific amino acid features and transcriptional regulation in response to hypoxia. Both groups of PCO proteins possess the ability to destabilize Cys-initiating proteins. Indeed, the inactivation of two A-type PCOs in Arabidopsis thaliana, PCO4 and PCO5, is sufficient to activate, at least partially, the anaerobic response in young seedlings, whereas the additional removal of B-type PCOs leads to a stronger induction of anaerobic genes and impairs plant growth and development. Our results show that both PCO types are required to regulate the anaerobic response in angiosperm. Therefore, while it is possible to distinguish two clades within the PCO family, separated by both amino acid features and transcriptional regulation, we conclude that they both contribute to restrain the anaerobic transcriptional program in normoxic conditions and together generate a molecular switch to toggle the hypoxic response in Arabidopsis.One sentence summary Hypoxic induction of Plant Cysteine Oxidases has been acquired and fixed in seed plants by ancestor proteins able to initiate the proteolysis of Cys-initiating protein substrates by the Arg/N-degron pathway.View Full Text