Two-step glycosylation of the contact site A protein of Dictyostelium discoideum and transport of an incompletely glycosylated form to the cell surface.
Two different types of oligosaccharides, designated type 1 and 2 carbohydrate residues, are present on the contact site A molecule, an 80-kDa glycoprotein involved in the formation of EDTA-stable cell adhesion during cell aggregation in Dictyostelium discoideum. The first precursor detected by pulse-chase labeling with [35S]methionine was a 68-kDa glycoprotein carrying type 1 carbohydrate. Conversion of the precursor into the 80-kDa form occurred simultaneously with the addition of type 2 carbohydrate. Tunicamycin inhibited type 1 glycosylation more efficiently than type 2 glycosylation. The first precursor detected in tunicamycin-treated cells by pulse-chase labeling was a 53-kDa protein lacking both carbohydrates, which was converted through addition of type 2 carbohydrate into a 66-kDa final product. Labeling of intact cells indicated that this 66-kDa glycoprotein is transported to the cell surface. Prolonged treatment with tunicamycin resulted in the accumulation within the cells of the 53-kDa precursor with no detectable exposure of this protein on the cell surface. It is concluded that type 1 carbohydrate, which is cotranslationally added in N-glycosidic linkages, is neither required for transport of the protein to the Golgi apparatus nor for type 2 glycosylation or protection of the protein against proteolytic degradation. Incapability of tunicamycin-treated cells of forming EDTA-stable cell contacts suggests a role for type 1 carbohydrate in cell adhesion. Type 2 carbohydrate is added posttranslationally. It is required in the absence of type 1 glycosylation for transport of the protein to the cell surface.
Hohmann
H P
HP
Max-Planck-Institut für Biochemie, Martinsried bei München, Federal Republic of Germany.
Bozzaro
S
S
Yoshida
M
M
Merkl
R
R
Gerisch
G
G
D000911:Antibodies, Monoclonal; D001693:Biological Transport, Active; D002241:Carbohydrates / Q000032:analysis; D003208:Concanavalin A / Q000378:metabolism; D004023:Dictyostelium / Q000032:analysis*; D005455:Fluorescent Antibody Technique; D006031:Glycosylation; D008970:Molecular Weight; D013499:Surface Properties; D014764:Viral Proteins / Q000378:metabolism*; D014909:Wheat Germ Agglutinins / Q000378:metabolism
D000911:Antibodies, Monoclonal; D002241:Carbohydrates; D014764:Viral Proteins; D014909:Wheat Germ Agglutinins; D003208:Concanavalin A