Post-translational modifications of the basic peroxidase isoenzyme from Zinnia elegans

The major basic peroxidase (ZePrx) from Zinnia elegans suspension cell cultures was purified and cloned. The purification resolved ZePrxs in two isoforms (ZePrx33.44 and ZePrx34.70), whose co-translational and post-translational modifications are characterized. Based on the N-terminal sequence obtained by Edman degradation of mature ZePxs, it may be expected that the immature polypeptides of ZePrxs contain a signal peptide (N-terminal pro-peptide) of 30 amino acids, which directs the polypeptide chains to the ER membrane. These immature polypeptides are co-translationally processed by proteolytic cleavage, and modeling studies of digestions suggested that the processing of the N-terminal pro-peptide of ZePrxs is performed by a peptidase from the SB clan (S8 family, subfamily A) of serine-type proteases. When the post-translational modifications of ZePrxs were characterized by trypsin digestion, and tryptic peptides were analyzed by reverse phase nano liquid chromatography (RP-nanoLC) coupled to MALDI-TOF MS, it was seen that, despite the presence in the primary structure of the protein of several (disulphide bridges, N-glycosylation, phosphorylation and N-myristoylation) potential post-translational modification sites, ZePrxs are only post-translationated modified by the formation of N-terminal pyroglutamate residues, disulphide bridges and N-glycosylation. Glycans of ZePrxs belong to three main types and conduce to the existence of at least ten different molecular isoforms. The first glycans belong to both low and high mannose-type glycans, with the growing structure Man_3–9(GlcNAc)₂. Low mannose-type glycans, Man_3–4(GlcNAc)₂, coexist with the truncated (paucimannosidic-type) glycan, Man₃Xyl₁Fuc₁(GlcNAc)₂, in the G₃ and G₄ sub-isoforms of ZePrx33.44. In ZePrx34.70, on the other hand, the complex-type biantennary glycan, Man₃Xyl₁Fuc₃(GlcNAc)₅, and the truncated (paucimannosidic-type) glycan, Man₃Xyl₁Fuc₁(GlcNAc)₂, appear to fill the two putative sites for N-glycosylation. Since the two N-glycosylation sites in ZePrxs are located in an immediately upstream loop region of helix F′′ (close to the proximal histidine) and in helix F′′ itself, and are flanked by positive-charged amino acids that produce an unusual positive-net surface electrostatic charge pattern, it may be expected that glycans not only affect reaction dynamics but may well participate in protein/cell wall interactions. These results emphasize the complexity of the ZePrx proteome and the difficulties involved in establishing any fine structure-function relationship.