Analysis of the soluble cell wall proteome of gymnosperms

We analyzed the cell wall proteome of lignifying suspension cell cultures (SCCs) from four gymnosperms that differ in evolution degree. This analysis showed the presence of “peptide sequence tags” (PSTs) corresponding to glucan endo-1,3-β-D-glucosidase, xyloglucan-endotrans-glucosylase/hydrolase, chitinases, thaumatin-like proteins and proteins involved in lignin/lignan biosynthesis, such as dirigent-like proteins and peroxidases. Surprisingly, and given the abundance of peroxidases in the cell wall proteome of these gymnosperms, PSTs corresponding to peroxidases were only detected in tryptic fragments of the cell wall proteome of Cycas revoluta. The current lack of knowledge regarding C. revoluta peroxidases led us to purify, characterize and partially sequence the peroxidases responsible for lignin biosynthesis in this species. This yielded three peroxidase-enriched fractions: CrPrx 1, CrPrx 2 and CrPrx 3. Analyses of tryptic peptides of CrPrx 2 (32 kDa) and CrPrx 3 (26 kDa) suggest that CrPrx 3 arises from CrPrx 2 by protein truncation, and that CrPrx 3 apparently constitutes a post-translational modification of CrPrx 2. That CrPrx 2 and CrPrx 3 are apparently the same enzyme was also deduced from the similarity between the kcat shown by both peroxidases for the three monolignols. These results emphasize the analogies between the cell wall proteome of gymnosperms and angiosperms, the complexity of the peroxidase proteome, and the difficulties involved in establishing fine structure–function relationships.