title: Transient N-acetylglucosamine in the biosynthesis of phytohemagglutinin: attachment in the Golgi apparatus and removal in protein bodies.
abstract: Cotyledons of the common bean (Phaseolus vulgaris L.) synthesize large amounts of the lectin phytohemagglutinin (PHA) during seed development. The polypeptides of PHA are synthesized by endoplasmic reticulum-bound polysomes and co-translationally glycosylated, pass through the Golgi complex, and accumulate in protein bodies, which constitute the lysosomal compartment in these cells. Some of the high-mannose sidechains of PHA are modified in the Golgi complex, and in mature PHA they contain N-acetylglucosamine, mannose, fucose, and xylose in the molar ratios 2, 3.8, 0.6, and 0.5. The results reported here show that the Golgi complex is also the site of additional N-acetylglucosamine incorporation into the modified sidechains. When developing cotyledons are labeled with [3H]glucosamine and glycopeptides of PHA present in the Golgi complex isolated, the radioactivity can be released as [3H]N-acetylglucosamine by digestion of the glycopeptides with beta-N-acetylglucosaminidase, indicating that the residues are in a terminal position. Arrival of PHA in the protein bodies is followed by the slow removal of these terminal N-acetylglucosamine residues, resulting in a decrease in the Mr of the modified sidechains. The biosynthetic intermediates of the glycoproteins destined for the lysosomal compartments of animal cells contain high-mannose sidechains modified by phosphate groups covered by N-acetylglucosamine that is labile to mild acid treatment. When cotyledons are labeled with [32P]orthophosphate, there is no radioactivity in PHA obtained from any of the subcellular fractions. There is also no release of radioactivity when [3H]glucosamine-labeled glycopeptides obtained from PHA in the Golgi complex are subjected to mild acid hydrolysis. These results indicate that the sorting-signals and posttranslational processing steps for proteins that are transported to the lysosomal compartment are different in plant cells and animal cells.
mesh_terms: D000117:Acetylglucosamine / Q000378:metabolism*; D000118:Acetylglucosaminidase / Q000378:metabolism; D002500:Centrifugation, Isopycnic; D002850:Chromatography, Gel; D005944:Glucosamine / Q000031:analogs & derivatives*; D006056:Golgi Apparatus / Q000378:metabolism*; D008361:Mannosidases / Q000378:metabolism; D017038:Mannosyl-Glycoprotein Endo-beta-N-Acetylglucosaminidase; D009940:Organoids / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism; D010835:Phytohemagglutinins / Q000096:biosynthesis*; D037121:Plant Lectins; D010944:Plants / Q000648:ultrastructure*; D013997:Time Factors; D043323:alpha-Mannosidase
chemical_list: D010710:Phosphates; D010835:Phytohemagglutinins; D037121:Plant Lectins; D008361:Mannosidases; D043323:alpha-Mannosidase; D000118:Acetylglucosaminidase; D017038:Mannosyl-Glycoprotein Endo-beta-N-Acetylglucosaminidase; D005944:Glucosamine; D000117:Acetylglucosamine