Molecular identification and characterization of an acidic peptide:N-glycanase from tomato (Lycopersicum esculentum) fruits*

Plant acidic peptide:N-glycanase (PNGase) is one of the deglycosylation enzymes and has been considered to be involved in the catabolism of glycoproteins in plant cells. However, the tangible physiological significance involved in plant differentiation or growth is yet unclear. In this study, as a first step to elucidate the physiological role of free N-glycans and the de-N-glycosylation machinery working in developing plant cells, we have succeeded in expressing a cDNA from tomato fruits in Pichia pastoris and identified an acidic peptide:N-glycanase in the culture supernatant. The PNGase-gene-encoded protein is a single polypeptide chain of 588 amino acids with a predicted molecular mass of 65.8 kDa. The deduced amino acid sequence showed 57.9% similarity with almond PNGase A. The recombinant tomato PNGase showed optimum activity at pH 4.5 and 40 degrees C. It did not require any metal ions for full enzymatic activity and could release the complex-type N-glycan from glycopeptides. Our phylogenetic analysis reveals that the plant acidic PNGase is completely different from the ubiquitous cytosolic PNGase and is involved in a different de-N-glycosylation mechanism associated with plant growth and development.