Peptide: N-glycanase (PNGASE) and Transglutaminase (TGASE): same catalytic domain, distint function.

Peptide:N-glycanase (PNGase) releases N-glycans from glycoproteins/glycopeptides, and this enzyme from bacteria and plants has been extensively used as a powerful “tool” reagent to study structure and biological functions of N-linked glycans on glycoproteins. The cytoplasmic PNGases (Png1p in yeast (1)), ubiquitously found throughout eukaryotes, are now widely recognized as a component implicated in the ERAD (ER-associated degradation) process, which constitute one of the quality control machineries for newly synthesized misfolded glycoproteins exported out of the ER lumen. Structurally, this enzyme is classified as “transglutaminase-super family” that contains a putative catalytic triad of amino acids (cysteine, histidine, and aspartic acid) (2). Interestingly, recent studies identified plant orthologue of the Png1 protein (AtPng1p) as transglutaminase, although these enzymes catalyzes totally different reaction (3). More recently, we have carbohydrate binding domain, which is presumable important for substrate recognition, through molecular modeling (4), Interestingly, critical residues predicted to be involved in carbohydrate binding is perfectly conserved in AtPng1p. In this study we have made several chimera proteins between AtPng1p and ScPng1p, and enzyme activities or carbohydrate-binding activity of these hybrid protreins are examined. Possible importance of amino acid replacement for conversion of these two enzyme activities will be discussed.