Intact chicken embryo neural retina cells have been shown to catalyze the transfer of galactose-14C from uridine diphosphate galactose (UDP-galactose) to endogenous acceptors of high molecular weight as well as to exogenous acceptors . Four lines of evidence indicate that the galactosyltransferases catalyzing these reactions are at least partly located on the outside surface of the plasma membrane : (a) there is no evidence for appreciable uptake of sugar-nucleotides by vertebrate cells nor did unlabeled galactose, galactose 1-phosphate, or UDP-glucose interfere with the radioactivity incorporated during the reaction ; (b) the cells remained essentially intact during the course of the reaction ; (c) there was insufficient galactosyltransferase activity in the cell supernatants to account for the incorporation of galactose-14 C into cell pellets ; and (d) the intact cells could transfer galactose to acceptors of 10 6 daltons, and the product of this reaction was in the extracellular fluid . Appropriate galactosyl acceptors interfered with the adhesive specificity of neural retina cells ; other compounds, which were not acceptors, had no effect . These results suggested that the transferase-acceptor complex may play a role in cellular recognition .Early theories on the mechanism of intercellular adhesion (11,12) proposed that apposing cell surfaces contained complementary molecules which acted as a lock and key, or, more precisely, like an antigen-antibody reaction . As a result of studies on mouse teratoma (4) and chicken embryo neural retina cells (8), we have recently extended this idea (7), suggesting that the complementary molecules are enzymes and substrates on the apposing cell surfaces, specifically, the complex carbohydrates and glycosyltransferases .The glycosyltransferases comprise several families of enzymes (6, 7), each of which catalyzes the following reaction :Sugar-nucleotide + oligosaccharide-acceptor sugar-oligosaccharide-acceptor + nucleotide Thus, chain elongation of the oligosaccharide units in the complex carbohydrates is effected by the addition of monosaccharide units through the action of different glycosyltransferases in a specific sequence . While all glycosyltransferases within one family, such as the sialyltransferases or the galactosyltransferases, utilize the same sugarnucleotide as the glycose donor (e .g ., cytidine monophosphate (CMP)'-sialic acid or uridine diphosphate (UDP)-galactose), each of the enzymes is specific for the acceptor molecule or its analogues . Thus, different galactosyltransferases are required to add galactose to the oligosaccharide 1 Abbreviations used in this paper : CMP, cytidine monophosphate ; TCA, trichloroacetic acid ; UDP, uridine diphosphate .5 3 6
Evidence is presented suggesting the presence of galactosyl transferases and galactosyl acceptors on the outer surfaces of intact Balb/c 3T3 cells. In addition, the data indicate that these transferases may only be capable of transferring galactose from uridine diphosphate galactose to galactosyl acceptors on adjacent cells after intercellular contact is made (trans-glycosylation). Intact Balb/c 3T12 cells, by contrast, show no requirement for intercellular contact in order to carry out this reaction suggesting that these cells, which do not exhibit contact inhibition of growth, may be able to transfer galactose to acceptors situated on the same cell as the enzyme (cisglycosylation).Electrophoretic and radioautographic assays were used to detect surface transferase activities in these two cell lines. Results of experiments on cells from sparse and dense cultures, and under conditions where intercellular contact was regulated, are consistent with the above hypothesis.Communication between adjacent cells occurs in embryonic, regenerating, and pathological tissue, but understanding these interactions on a molecular level remains one of the central problems of developmental biology. Attempts to isolate specific "informational" molecules whose passage from cell to cell might account for such phenomena as embryonic induction and growth control between normal cells have yielded equivocal results (1, 2). An alternative model for intercellular communication is suggested by the data reported in this paper.Our results indicate that the established cell lines Balb/c 3T3 and Balb/c 3T12 from mouse embryos possess galactosyl transferases on their cell surfaces, as do at least two other cell types (3, 4). In addition, evidence is presented that suggests that the highly contact-inhibited 3T3 cells catalyze the transfer of galactose from uridine diphosphate galactose to acceptors on adjacent cells ("trans"-glycosylation), whereas the highly malignant and relatively non-contact-inhibited 3T12 cells can catalyze this transfer to acceptors located on the same cell ("cis"-glycosylation).The glycosyl transferases transfer single glycose units from glycose donors (nucleotide sugars) to glycose acceptors (nonreducing termini of oligosaccharides). Each enzyme shows a high degree of specificity for a particular donor, as well as for a particular acceptor. The transferases catalyze the following general reaction (5). Nucleotide-X + Y-Z _ nucleotide + X-Y-Z, where X, Y, and Z are single glycose units. Except for the sialyltransferases, manganese is a required cofactor for all of these enzymes (5).Recent work has shown that glycosyl transferases may be present on the outer surface of plasma membranes of cells of the chick-embryo neural retina (3) and human blood platelets 485 (4). In the case of the neural retina (3), evidence has been presented suggesting that the interaction of these surface enzymes with their appropriate substrates on adjacent cells is at least partially responsible for the adhe9fie recognition demonstrated by th...
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