The signal for Golgi retention of bovine beta 1,4-galactosyltransferase is in the transmembrane domain.
The expression and localization of bovine beta 1,4-galactosyltransferase (Gal T) has been studied in mammalian cells transfected with Gal T cDNA constructs, and the role of the amino-terminal domains of Gal T in Golgi localization examined. Here we demonstrate that the transmembrane (signal/anchor) domain of bovine Gal T contains a positive Golgi retention signal. Bovine Gal T was characterized in transfected cells with anti-bovine Gal T antibodies, affinity-purified from a rabbit antiserum using a bacterial recombinant fusion protein. These affinity-purified antibodies recognized native bovine Gal T and showed minimum cross-reactivity with Gal T from non-bovine sources. Bovine Gal T cDNA was expressed, as active enzyme, transiently in COS-1 cells and stably in murine L cells, and the product was shown to be localized to the Golgi complex by immunofluorescence using the polypeptide-specific antibodies. A low level of surface bovine Gal T was also detected in the transfected L cells by flow cytometry. The removal of 18 of the 24 amino acids from the cytoplasmic domain of bovine Gal T did not alter the Golgi localization of the product transiently expressed in COS-1 cells or stably expressed in L cells. Both the full-length bovine Gal T and the cytoplasmic domain deletion mutant were N-glycosylated in the transfected L cells, indicating both proteins have the correct N(in)/C(out) membrane orientation. Deletion of both the cytoplasmic and signal/anchor domains of bovine Gal T and incorporation of a cleavable signal sequence resulted in a truncated soluble bovine Gal T that was rapidly secreted (within 1 h) from transfected COS-1 cells. Replacement of the signal/anchor domain of bovine Gal T with the signal/anchor domain of the human transferrin receptor resulted in the transport of the hybrid molecule to the cell surface of transfected COS-1 cells. Furthermore, a hybrid construct containing the signal/anchor domain of Gal T with ovalbumin was efficiently retained in the Golgi complex, whereas ovalbumin anchored to the membrane by the transferrin receptor signal/anchor was expressed at the cell surface of transfected COS-1 cells. Overall, these studies show that the hydrophobic, signal/anchor domain of Gal T is both necessary and sufficient for Golgi localization.
Teasdale
R D
RD
Department of Pathology and Immunology, Monash University Medical School, Melbourne, Victoria, Australia.
D'Agostaro
G
G
Gleeson
P A
PA
D000595:Amino Acid Sequence; D000818:Animals; D000906:Antibodies / Q000276:immunology; D001483:Base Sequence; D015153:Blotting, Western; D002240:Carbohydrate Sequence; D002417:Cattle; D002460:Cell Line; D004247:DNA / Q000235:genetics; D004926:Escherichia coli / Q000201:enzymology; D005434:Flow Cytometry; D005455:Fluorescent Antibody Technique; D005700:Galactosyltransferases / Q000235:genetics / Q000276:immunology / Q000378:metabolism*; D005982:Glutathione Transferase / Q000378:metabolism; D006031:Glycosylation; D006056:Golgi Apparatus / Q000378:metabolism*; D000882:Haplorhini; D007425:Intracellular Membranes / Q000201:enzymology*; D007739:L Cells; D051379:Mice; D008969:Molecular Sequence Data; D009154:Mutation; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000276:immunology / Q000378:metabolism; D014162:Transfection
D000906:Antibodies; D011993:Recombinant Fusion Proteins; D004247:DNA; D005700:Galactosyltransferases; D005982:Glutathione Transferase