Ehrlich ascites tumor cell UDP-Gal:N-acetyl-D-glucosamine beta(1,4)-galactosyltransferase. Purification, characterization, and topography of the acceptor-binding site

Galactosyltransferase--still up and running

The following review on galactosyltransferase (gal-T1) intends to cover genetic, biochemical, structural, biotechnological, cell biological and medical aspects of this enzyme in a comprehensive manner from discovery to the present day which have brought to light a genetic defect of this enzyme. Early work has only been included if it appeared relevant to ongoing issues. Following the evolution of a research topic over 40 years is in itself a fascinating endeavor as it permits to observe the ins and outs of hypotheses, fashions and errors. Gal-T1 is a beautiful example as it has been involved in almost every aspect of life science. Importantly, there is a future to this enzyme as a research topic, since many questions still remain unanswered: to which extent is it a representative Golgi protein? What is the role of the gene family of gal-Ts? Does gal-T1 exert any functions other than a catalytic one? Why is it phosphorylated? Does it form homodimers in vivo? Surely, there is room for further work, which is likely to reveal further insights into cellular trafficking and signaling and, in the context of the gene family, shall contribute to understanding development and morphogenesis.

Subcellular distribution of terminal alpha-D- and beta-D-galactosyl residues in Ehrlich tumour cells studied by lectin-gold techniques

We have studied by high resolution in situ light and electron microscopic lectin-gold techniques the subcellular distribution of alpha-D-Gal residues using the Griffonia simplicifolia I-B4 isolectin and compared it with that of beta-D-Gal residues as detected with the Datura stramonium lectin in Ehrlich tumour cells grown as ascites or monolayer. The microvillar but not the smooth plasma membrane regions were labelled with the Griffonia simplicifolia I-B4 isolectin whereas both plasma membrane regions were equally well labelled with the Datura stramonium lectin. Elements of the endocytotic/lysosomal system such as coated membrane invaginations and vesicles, early and late endosomes and secondary lysosomes were positive for both alpha-D-Gal and beta-D-Gal residues. A particular feature of Ehrlich tumour cells is an elaborate tubular membrane system located in the pericentriolar region which is labelled throughout by both lectins and represents part of the endosomal system. In the Golgi apparatus labelling with both lectins was observed to commence in trans cisternae which is indirect evidence for a joint distribution of the sequentially acting beta 1,4 and alpha 1,3-galactosyl-transferases.

Colonic cancer cell (HT29) adhesion to laminin is altered by differentiation: adhesion may involve galactosyltransferase

The effects of cell differentiation on cell adhesion to laminin were studied using the human colon tumor cell line, HT29. HT29 cells were induced to differentiate either by glucose deprivation (HT29glc- vs HT29glc+) or by 2 mM butyrate (HT29glc-+B+). Adhesion was assayed after incubating cell suspensions in microtiter wells previously coated with laminin or other substrates. HT29glc+ cells adhered preferentially to laminin over BSA, fibronectin, and ovalbumin. The adhesion to laminin was greater than 50% of maximum within 15 min. HT29glc- cell adhesion to laminin was consistently lower than that for HT29glc+ or HT29glc+B+ cells. alpha-Lactalbumin (ALA), a modifier of galactosyltransferase (GT) substrate specificity, caused a significant reduction (greater than 50%) in HT29glc+ cell adhesion to laminin when ALA was added to the adhesion incubation mixture. Addition of glucose+ALA to the suspension restored adhesion to laminin. Ovalbumin, a GT substrate, increased adhesion of HT29glc+ and HT29glc- cells to laminin, but lactose, a GT product, did not. The data show that undifferentiated HT29 cells adhere preferentially to laminin over fibronectin and collagen IV and that differentiation of HT29 cells reduces adhesion to laminin. In addition, the data imply that cell adhesion to laminin may be mediated by factors that also modify galactosyltransferase activity.

Characterization of a murine beta 1-4 galactosyltransferase expressed in COS-1 cells

We inserted a full-length murine cDNA, which had been isolated from F9 embryonal carcinoma cells by using a bovine lactose synthetase A protein cDNA as a probe, in a mammalian expression vector (pCMGT1) and expressed it in COS-1 cells to characterize the pCMGT1-directed enzyme. The galactosyltransferase activity toward asialo-agalacto-transferrin (AsAg-Tf) in the pCMGT1-transfected cells was approximately eightfold higher than that in mock- or non-transfected cells. In contrast, no difference was observed in the specific activity of galactose transfer between pCMGT1-transfected cells and mock- or non-transfected cells when asialo-ovine submaxillary mucin were used as an acceptor. Since almost all [3H]galactose incorporated into the AsAg-Tf was released by digestion with streptococcal beta-galactosidase, most of the linkage created by this enzyme was in the Gal beta 1-4GlcNAc group. The acceptor specificity of the pCMGT1-directed enzyme was changed from N-acetylglucosamine to glucose by adding alpha-lactalbumin in the reaction mixture. Alpha-Lactalbumin also partially inhibited the galactose transfer to AsAg-Tf. The kinetic study revealed that the apparent Km values of the pCMGT1-directed enzyme for N-acetylglucosamine, AsAg-Tf and UDP-Gal are 2 mM, 60 microM and 24 microM, respectively. These results indicated that the murine cDNA isolated from F9 cells encodes an active enzyme which catalyzes not only the lactose synthesis but also the transfer of galactose to N-acetylglucosamine residues of Asn-linked sugar chains of glycoproteins in a beta 1-4 linkage.

Localization of glycosylation sites in the Golgi apparatus using immunolabeling and cytochemistry

This review summarizes data on the distribution of certain glycosylation steps in the Golgi apparatus as revealed by immunolabeling and lectin techniques. The methodical basis for such investigations was provided by the introduction of the colloidal gold marker system for immunolabeling and the development of new means of tissue processing such as the low-temperature embedding technique using Lowicryl K4M. The application of these techniques together with highly specific antibodies has provided much of the basis for our current understanding of the Golgi apparatus in functional terms. Thus, in many cell types, three Golgi apparatus compartments can be distinguished, whereas in others no such functional subdivision is evident. Investigations on sialyltransferase distribution have also provided direct evidence that GERL is structurally and functionally part of the Golgi apparatus.