Abstract
Plant glycoproteins such as extensin and arabinogalactan proteins are modified with plant specific O-linked galactosylation on serine and hydroxyproline (Hyp) residues. An assay system for peptidyl serine O-galactosyltransferase (SGT) activity was established, using UDP-galactose as a donor, and chemically synthesized peptides as an acceptor in Chlamydomonas reinhardtii cell extracts. SGT protein was purified from cell extracts of C. reinhardtii, and subjected to Mascot analysis of proteins to determine the amino acid sequences by mass spectrometry. The analyzed peptide sequences were completely matched with an ORF in the C. reinhardtii genome database. The SGT activity was confirmed through the expression system in yeast Saccharomyces cerevisiae, therefore, designated as CrSGT1. Highly homologous ORFs were found in various plant genomes, such as Arabidopsis thaliana, Nicotiana tabacum, etc. The putative AtSGT1 and NtSGT1 also showed SGT activity when expressed in yeasts. The SGT proteins contained a conserved DXD motif, but no homology with known glycosyltransferases, indicating that SGT1 is a novel glycosyltransferase gene family, existing only in plant kingdom.
