Structures and Functions of the Sugar Chains of Cell

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1 CELL STRUCTURE AND FUNCTION 4, (1979) C by Japan Society for Cell Biology Structures and Functions of the Sugar Chains of Cell Surface Glycoproteins* Akira Kobata Department of Biochemistry, Kobe University School of Medicine, Ikuta-ku, Kobe 650, Japan ABSTRACT. Asparagine-linked sugar chains of glycoproteins can be classified structurally into three groups; the high mannose type, the complex type and the hybrid type. The occurrence of common core structure in each group is accounted for by the unique biosynthetic pathway of the asparaginelinked sugar chains. Accumulating evidence showing that the outer chain moieties of the complex type sugar chains have structural variations suggests that they play key roles in cellular recognition processes. The plasma membranes of animal cells.contain a great variety of glycoproteins and glycolipids. Evidence that shows that the carbohydrate moieties of these glycoconjugates are involved in various cellular recognition phenomena present in multicellular organisms is accumulating. Early in 1964, Gesner and Ginsburg (1) studied the lymphocyte homing mechanism and found that exoglycosidase digestion deprives lymphocytes of their ability to pass through the epithelial cells. Since then, many important processes, such as specific cellular adhesion (2-4), histogenetic aggregation of cells (5-9), and sexual mating of procaryotic cells (10-11) have been shown to include membrane glycoconjugates. The molecular bases of these complex recognition phenomena are unknown. However, recent development in studies of the clearance mechanism of various serum glycoproteins (12, 13), and of the uptake of lysosomal enzymes by fibroblasts (14-16) has revealed many animal lectins in the plasma membrane that specifically bind to definite structures of the sugar chains. This suggests that the specific binding of membrane proteins with the sugar chains of the glycoconjugates of the plasma membrane of opponent cells, analogous to the binding mechanism of isoagglutinin to blood group determinants (17, 18), may be the initial key event in cellular interactions. Alteration of the sugar chain structure of the plasma membrane glycoconjugates has been widely observed in cultured cells transformed by viral, chemical and spontaneous methods (19-21). This may also be the biochemical basis of the transformed state that is free of the various controls that operate in normal cells. Structural study of the sugar chains of plasma membrane glycoconjugates is, therefore, now one of the most popular fields of biochemistry and cell biology. In this review, I will discuss current knowledge concerning the structure of the carbohydrate moieties of glycoproteins, and the progress made in the study of fibronectin, *This work has been supported in part by research grants from the Mitsubishi Foundation and by Scientific Research Funds from the Ministry of Education, Science and Culture of Japan, and from the Cancer Division, Public Health Bureau, the Ministry of Health and Welfare, Japan. 169

2 170 A. Kobata TABLE 1. MUCIN-TYPE SUGAR CHAINS IN PLASMA MEMBRANE GLYCOPROTEINS that is attracting widespread interest because of the diverse biological action of fibronectin in cell to cell interaction. I. Structural characteristics of plasma membrane glycoproteins. Most plasma membrane glycoproteins have a common structural characteristic in that they have a portion enriched in hydrophobic amino acids (23). This hydrophobic portion is embedded deep inside the phospholipid bilayer of the plasma membrane stretching the hydrophilic portion with sugar chains outside the cell surfaces (24). The sugar chains of the plasma membrane glycoproteins can be structurally classified into two groups (25). Those linked to the polypeptide chain through GalNAc, Thr and Ser groupings have been given the trivial name mucin-type sugar chains, because they were found as the major carbohydrate moieties of mucins secreted from mucous glands. Table 1 shows some of the typical mucin-type sugar chains found in plasma membrane glycoproteins. The second grouping is composed of sugar chains linked through the GlcNAc Asn grouping. Although the structures of the asparagine-linked sugar chains of plasma membrane glycoproteins are mostly unknown, TABLE 2. HIGH MANNOSE-TYPE SUGAR CHAINS

3 Cell Surface Glycoproteins 171 TABLE 3. COMPLEX-TYPE SUGAR CHAINS studies of the structures of many glycoproteins in the body fluid have indicated that they can be classified into at least three subgroups (26, 27). The first is the high mannose type sugar chains composed only of mannose and N-acetylglucosamine. They have a heptasaccharide, Manal 6 (Mana1,3)Mana 1 6(Mana 1 3)Mang 1.4G1cNAci31,4G1c- NAc as a common inner core structure (Table 2). The second is the complex type sugar chains. They have a common core structure; Manal.6(Manal 3)ManƒÀl 4G1cNAcƒÀ1 4G1cNAc+Asn, to which two to four outer chains composed of sialic acid, galactose and N-acetylglucosamine are attached at two non-reducing terminal a-mannosyl residues. In many cases, an a-fucosyl residue TABLE 4. HYBRID-TYPE SUGAR CHAINS

4 172 A. Kobata is linked at the C-6 position of the proximal N-acetylglucosamine residue of the core portion. Some typical complex type sugar chains are shown in Table 3. The third is the hybrid type sugar chains recently found through study of hen egg albumin. They contain the same trimannose-di-n-acetylglucosamine core found in complex type sugar chains. However, one branch of this core has substitutions of additional mannose residues as in the high mannose type, but the other branch carries part of the sialyl-galactosyl-n-acetylglucosamine trisaccharide sequence (Table 4). Another common characteristic of the hybrid type sugar chains is the presence of a f3-n-acetylglucosamine residue at the C-4 position of the branching fl-mannosyl residue. Our present knowledge of these asparagine-linked sugar chains was obtained from the study of glycoproteins found in body fluid the asparagine-linked sugar chains of plasma membrane glycoproteins are believed to have similar structures, because the biosynthetic scheme of asparagine-linked sugar chains determined through the study of a plasma membrane glycoprotein produces the common core structures found in the high mannose and complex type sugar chains as described in the next section. H. Biosynthesis of the asparagine-linked sugar chains of glycoproteins. The presence of common cores in the three subgroups of the asparagine-linked sugar chain suggested that they are formed by a strictly regulated biosynthetic pathway. A comprehensive picture of this pathway has recently been drawn based on the cummulative work of many laboratories (28-32). To make a long story short, a tetradecasaccharide, bound through a pyrophosphate linkage to a polyisoprenol, dolichol, is formed by the stepwise transfer of monosaccharides from nucleotide sugars (Fig. 1). This tetradecasaccharide is then transfered en bloc to the asparagine residue of polypeptide chains. All the glucose residues and a part of the mannose residues of the sugar chain are then hydrolytically removed by the action of glycosidases tightly bound to the Golgi membrane. The trimming process is somewhat complicated as summarized in Fig. 2. This scheme indicates that the high mannose type sugar chains are biosynthetic intermediates rather than final products. Several glycoproteins such as bovine rhodopsin (33) and pancreatic ribonuclease (34) are known to contain only the high mannose type sugar chains. Tissues which produce this glycoprotein may lack the glycosyltransferases responsible for the formation of the outer chain moiety of the complex type sugar chains. From this step of the biosynthetic pathway in Fig. 2, the pathway leading to the hybrid type sugar chains is branched, and requires future study. In any event, the biosynthetic pathway indicates that the asparaginelinked sugar chains that fall into the complex and hybrid types are final products that play key roles in cellular recognition phenomena. If that is the case, the sugar chains of these two types would have structural variations in their outer chain moieties corresponding to the variety of the cellular recognition process. As shown in Table 3, the complex type sugar chains reported so far have very similar structures, although variation has been observed in the number of the outer chain trisaccharide, NeuAca2, 6GalƒÀ 4G1cNAc, and its stage of completion. Our recent studies, however, have indicated that the outer chains are not that simple but are more complex in many purified glycoproteins. As an example of this variation the study of fibronectin will be described. III. Structure of fibronectin. In 1948, Morrison et al. (35) isolated a glycoprotein from human plasma. This glycoprotein was named the cold insoluble globulin, becouse it specifically binds with

5 Cell Surface Glycoproteins Fig. 1. Pathway of asparagine-linked sugar chain formation. Dol-P, dolicholphosphate; Dol-P-P, dolicholpyrophosphate. fibrinogen, blood coagulation factor VIII, and factor XIII, to form an insoluble aggregate at low temperatures (36-38). The important physiological role of this glycoprotein came to the fore when its close relation to cell surface fibronectin (in this paper called cellular fibronectin) became evident (39). Cellular fibronectin is a major glycoprotein constituting the plasma membrane in many cultured cells, and has an apparent subunit molecular weight of 220,000 on SDS-polyacrylamide gel electrophoresis (40, 41). This glycoprotein has become the object of much attention since it was found that the amount of cellular fibronectin mostly decreases when cells are

6 174 A. Kobata Fig. 2. Processing pathway of asparagine-linked sugar chains of glycoproteins. transformed by oncogenic viruses or by carcinogens (42-46). Studies to determine the function of fibronectin revealed that it plays a vital role in cell-substratum adhesiveness, which is related to the shape and behavior characteristics of normal cultured cells (47-49). Addition of fibronectin to cultures of transformed fibroblasts resulted in cell aggregation (50-52). This aggregation seems to be mediated by the calcium ion since the removal of this ion from the culture medium completely destroys the effect of fibronectin. In addition to its morphological effects on transformed cells,

7 Cell Surface Glycoproteins 175 fibronectin seems to restore the normal social behavior of these cells. Transformed fibroblasts have the tendency to grow by piling up on other cells, but normal fibroblasts show neat alignment in culture and form a monolayer. Addition of fibronectin restores this normal social alignment in many transformed cells (49, 50). Why two forms of fibronectin (cellular and plasma) occur has yet to be determined. Since cultured cells tend to release part of their fibronectin into the culture medium (53, 54). plasma fibronectin may well be the released form of cellular fibronectin. Many groups have studied the structure of fibronectin in order to establish the molecular basis of these interesting functions. Plasma fibronectin seems to be composed of elongated glycoproteins as depicted in Fig. 3. It is a dimer of two subunit polypeptides of 220,000 daltons connected by several disulfide bridges (55, 56). Cellular fibronectin also may occur as multimers. The disulfide linkages seem to be important in the physiological function of fibronectin, since the subunit monomer of fibronectin did not increase the adhesion of transformed cells to the substratum (57). The monosaccharide composition of plasma fibronectins show that both glycoproteins contain glucosamine, mannose, galactose and sialic acid (58, 59). Since no galactosamine was detected, both fibronectins should contain asparagine-linked sugar chains but not the mucin-type sugar chain. The structure of the asparaginelinked sugar chains of fibronectin has recently been reported from three laboratories. Wrann studied the structure of the sugar chain of human plasma fibronectin using only methylation analysis (60). He speculated that the inner core portion of the sugar chain has the same pentasaccharide structure, Mana1 6(Manal 3)Man1ƒÀ 4G1c- NAcƒÀ1 4G1cNAc Asn, found in other complex type asparagine-linked sugar chains, and proposed the biantennary structure shown in Fig. 4A. We have studied the complete structures of the sugar chains of bovine serum fibronectin using the hydrazinolysis technique, and have determined the structures of the released oligosaccharides, by a combination of sequential exoglycosidase digestion and methylation analysis. The presence of the four structurally different sugar chains (Fig. 4B) in this glycoprotein was confirmed through this study (61). The yield of total oligosaccharides indicated that one fibronectin subunit has three asparagine-linked sugar chains. These sugar chains can basically be classified as the complex type. However, three characteristic new features were found in these sugar chains. The first is the presence of a Gal Â1?3G1cNAc grouping in the outer chain moieties of A-3 and A-4. GalƒÀ 3G1c- NAc and Gal1ƒÀ,4G1cNAc groupings were originally found in the immunological determinants of A, B and H blood type antigens, and have been named Type I and Type II chains, respectively. Mucin-type sugar chains contain both groupings. However, the asparagine-linked sugar chains reported so far contain only the Type II chain in their outer chain moieties (Table 2). We recently found that bovine prothrombin also has the Type I chain (62). Since prothrombin has been reported to be one of the serum factors essential for the growth of cultured cells, the presence of the Type I chain in both glycoproteins may indicate that they act on the cell surfaces through Fig. 3. A model of bovine plasma fibronectin.

8 176 A. Kobata A B. C. Fig. 4. Proposed structures of the carbohydrate moieties of three fibronectin preparations. this unique sugar chain. The second characteristic feature of the sugar chains of bovine fibronectin is the amount of sialic acid residues. In contrast to the Type II chain, which attaches only one sialic acid residue, the Type I chain can accept two sialic acid residues (Fig. 4B). Therefore, the complex type sugar chains with a Type I chain in their outer chain moiety gives the glycoprotein an extraordinarily high sialic acid content. The third is the presence of the NeuAca2 4Gal grouping, never before found in the sugar chains of glycoproteins. The presence of this grouping in human plasma fibronectin has been suggested by Wrann also (60). In almost all the complex type sugar chains reported so far, sialic acid is linked at the C-6 positions of the terminal galactoses. Quite recently, we found that all the sialic acid residues of the asparaginelinked sugar chains of human chorionic gonadotropin are linked at the C-3 position of the galactose (63). The diversity of the sialyl linkages in the complex type sugar chains indicates not only different distribution of sialyl transferases among the tissues, b ut the possibility that some sialic acid residues of the glycoproteins play important roles in recognition phenomena as well as in endowing glycoprotein molecules with acidity. The limitation in the amount of sample available has hampered the study of cellular fibronectin. However, by improving the extraction method of plasma membrane glycoproteins, Carter and Hakomori succeeded in isolating enough cellular

9 Cell Surface Glycoproteins 177 fibronectin for the structural study of the sugar portion from cultured hamster embryo fibroblasts (64). After exhaustive pronase digestion, a glycopeptide with a molecular weight of 2,000 was isolated from the glycoprotein. Analysis of the monosaccharide composition incidated that the glycopeptide has one fucose, mannoses, two galactoses and approximately four N-acetylglucosamines in one molecule (59). Recently, Carter and Hakomori proposed the structure of the sugar moiety of the glycopeptide as shown in Fig. 4C (65). The NeuAca2 4Gal and GalƒÀ1 3G1cNAc groupings found in bovine plasma fibronectin are not included in this sugar chain. Moreover, the sialic acid content of their glycopeptide preparation was far smaller than that of bovine and human plasma fibronectins. The structural varieties of the three fibronectin preparations may indicate that the sugar chains of fibronectin differ structurally among different species. It has already been reported that the plasma membrane receptors of the liver parenchymal cells responsible for the hepatic clearance of serum glycoproteins recognize the different structures of sugar chains among different species (66). Therefore, species difference must also be considered in studying the physiological role of the carbohydrate moiety of fibronectin. IV. Concluding remarks In 1965, Eylar advocated a theory that suggested that the carbohydrate portions of glycoproteins act as signals for extracellular secretion. The finding of many simple proteins in extracellular fluid cast doubt on this theory, and it was completely negated when the actual secretion of sugar chain-free interferon from L-cells cultured in the presence of tunicamycin was observed (67). The antibiotic specifically inhibits the formation of the asparagine-linked sugar chains. The signal theory, however, has survived in a disguised form. Both the biological phenomena described in my introduction, and many other lines of evidence indicate that sugar chains may act as signals (68-74). The development of chemical techniques to study glyco conjugates may provide the tools to determine the biochemical basis of these complicated processes in multi-cellular organisms in the near future. Acknowledgment. I wish to express my gratitude to Miss J. Fujii for her skillful secretarial assistance. REFERENCES 1. GESNER, B. M. and V. GINSBURG. Effect of glycosidases on the fate of transfused lymphocytes. Proc. Natl. Acad. Sci. U.S.A. 52, , OPPENHEIMER, S. B., M. EDIDIN, C. W. ORR and S. ROSEMAN. An L-glutamate requirement for intercellular adhesion. Proc. Natl. Acad. Sci. U.S.A. 63, , ROTH, S., E. J. McGUIRE and S. ROSEMAN. An assay for intercellular adhesive specificity..1. Cell. Biol. 51, , CHIPOWSKI, S., Y. C. LEE and S. ROSEMAN. Adhesion of cultured fibroblasts to insoluble analogues of cell surface carbohydrates. Proc. Natl. Acad. Sci. U.S.A. 70, , LILIEN, J. E. Current Topics in Developmental Biology, Vol. 4, ed. A. MONROY and A. A. MOS- CONA, Academic Press, New York, pp , KLEINSCHUSTER, S. J. and A. A. MOSCONA. Interaction of embryonic and fetal neural retina cells with carbohydrate-binding phytoagglutinins : cell surface changes with differentiation. Exp. Cell Res. 70, , MOSCONA, A. A. EMBRYONIC and neoplastic cell surfaces : availability of receptors for concanavalin A and wheat germ agglutinin. Science 171, , WEISER, M. M. Concanavalin A agglutination of intestinal cells from the human fetus. Sciecne 177, ,1972

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