The complexity of mucins

Biochirnie 70 (1988) 1471 - 1482 1471 © Soci6t6 de Chimie biologique/Elsevier, Paris

The complexity of mucins

Philippe ROUSSEL, Genevieve LAMBLIN, Michel LHERMITTE, Nicole HOUDRET, Jean-Jacques LAFITTE, Jean-Marc PERINI, Andr6 KLEIN and Andr6e SCHARFMAN

INSERM U16, pl. de Verdun, 59045 Lille Cedex, France

(Received 13-4-1988, accepted 17-5-1988)

S u m m a r y - - Mucins represent the main components of gel-like secretions, or mucus, secreted by mucosae or some exocrine glands. These high-molecular-weight glycoproteins are characterized by the large number of carbohydrate chains O-glycosidically linked to the peptide.

The determination of mucin molecular weight and conformation has been controversial for several reasons: 1) the methods used to solubilize mucus and to purify mucins are different and 2) the molecules have a strong tendency to aggregate or to bind to other molecules (peptides or lipids). Recently, electron microscopy has shown the filamentous shape of most mucins and their polydisperse character which, in some secretions, might correspond to a polymorphism of the peptide part of these molecules.

The recent development of high pressure liquid chromatography and high-resolution proton NMR spectroscopy has allowed major progress in the structural study of mucin carbohydrate chains. These chains may have from 1 to about 20 sugars and bear different antigenic determinants, such as A, B, H, I, i, X, Y or Cad antigens. In some mucins, such as human respiratory mucins, the carbohydrate chain diversity is remarkable, which raises many questions.

Mucins are molecules located at the interface between mucosae and the external environment. The carbohydrate chain diversity might allow many interactions between mucins and microorganisms and play a major role in the colonization or the defense of mucosae.

mucins / glycoproteins / mucus glycoproteins / mucin molecular weight / mucin electron microscopy / O-gly- cans / carbohydrate chains

I n t r o d u c t i o n

The word mucin, which is an ancient term in the field of glycoconjugates, has proven to be very persistent. For more than a century, the concept of mucin has been associated with material secreted in mucus or in pathological cysts and mucins were probably the first type of com- pound to be clearly recognized as glycoproteins.

Today, one can define a mucin, or a mucus glycoprotein, as a glycoprotein with a high molecular weight, from several hundred thousand to several million daltons, containing hundreds of carbohydrate chains, attached to the peptide by O-glycosidic linkages between N-acetylgalactosamine and a hydroxylated amino acid (serine or threonine). The usual representation of a mucin is that of a 'bottle-brush' (for recent general

reviews, see [1-41). In addition to N-acetylgalactosamine, mucins may contain galactose, N-acetylglucosamine, fucose, various sialic acids [5] and sulfate. The peptide part is rich in threonine, serine, proline, glycine and alanine.

This definition applies mainly to glycoproteins secreted by exocrine glands or by mucosae. However, 'mucin-like' molecules, isolated from the surface of certain cancer cells, also seem to fulfill these criteria [6-9]. Behind this broad definition, one should recognize two facts: 1) there is a lot of controversy with regard to the precise Mr and conformation of these molecules; 2) the carbohydrate moieties of mucins secreted by certain mucosae or certain cysts may be very diverse.

The present review, which will be limited to the secreted mucins, will mainly concern those two aspects.

1472 Philippe Roussel et al.

Conformation o f mucins

Problems raised by the purification o f mucins

Since, in most secretions, mucins are part of a mucus gel with theological properties, their purification needs, first of all, the solubilization of the mucus gel. Various procedures have been used: proteolytic enzymes [2,i0], disulfide bond- breaking agents [2, 11], mechanical agitation after water dilution [12] and dissociating agents [13-15].

Once the gel is solubilized, the mucins may be purified by different methods. Two of them are widely used: gel chromatography in dissociating agents [13, 15] and density gradient centrifuga- tion in cesium salts which was introduced into the mucin field by Creeth and Denborough [16] and then used by others [17, 2].

The purification of mucins is difficult: the molecules may not be intact; they have a strong tendency to bind various peptides or lipids and also to self-associate. Therefore, in the compari- son of different mucin preparations, one always has to take into consideration the various steps in the purification procedures.

The molecular weight and general confor- motion "~¢ . . . . ;~" "

In the seventies, different models were proposed for various mucins [18, 19]. The 'windmill' model, proposed by Adrian Allen for pig gastric mucins, was an early interesting model [20]. The Mr was estimated to be in the order of 2 × 106 kDa; due to the effects of reducing agents, four glycosylated subunits and a link-protein covalent- ly attached by disulfide bonds could be defined; each subunit comprised a highly glycosylated region resistant to proteases and a 'naked' region, devoid of carbohydrate chain and containing most of the disulfide bonds.

The estimation of mucin Mr is usually very difficult and controversial. Table I lists some of the Mr values reported in the literature for different mucin preparations [21-38]. There are large differences according to the method used. Ovine submaxillary mucin (OSM) is in the range of 0.5 × 106 with a sedimentation equilibrium and between 0.8 and 5 × 106 with light scattering. For the other mucins, the values obtained by sedimentation equilibrium (between 1 and 5 × 106) are lower than those obtained by light scattering. It is possible that sedimentation equi-

Table !. Mr of different mucins (x 106 Da).

Mucin Sedimentation equilibrium

so, D o Light scattering

Ovine submaxillary 0.4-0.5 [21] Bovine submaxillary 0.4-0.5 [21]

Pig submaxillary 2.8 [21 ]

Pig gastric 2.3 [26] Human gastric 1.8-2.2 [29] Sheep small intestine 5 [30] Pig small intestine 1.7 [311 Rat small intestine Pig colonic Rat colonic 0.9 [34] Bovine cervical Monkey cervical 1 [36] Human cervical Human respiratory 3.3-7.7 [37]

1 [381

2.3

1.8 15

[27]

[32] 133]

0 .8- 5.2 [221 4 123] 1.96- 2.75 [24] 2.1-11.4 I241

1-20 I251 44 [281

16.4 [351

11 [281 18 [28]

The complexity of mucins 1473

iiblium nlay miss the largest species of a polydisperse population and that light scattering may be influenced by aggregates.

In recent years, there have been several appli- cations of electron microscopy (EM) to the exa- mination of mucin architecture. In this field, one should mention the pioneer role of Henry Slay- ter, from Boston, who, as early as 1974, had observed the filamentous nature of ovarian cyst mucins [391.

Since then, different mucin preparations have been analyzed by electron microscopy. Most mucins appear as polydisperse, linear and apparently flexible threads (Fig. 1). These threads have been observed in human respiratory [15, 40-42], pig gastric [43], human cervical [43] and ovine submaxillary [44] mucins: their lengths vary over a broad range (Table II).

It is useful to attempt a rough correlation of the length measurements with molecular weight. One can take the example of the smallest mucin that we know: ovine submaxillary mucin. Using sedimentation equilibrium in 0.5 M NaCI, Hill's group found a value of about 0.5 x 106 Da [45]. After removal of the sialic acid and N-acetylgalactosamine residues which constitue the carbohydrate chains, the molecular weight of the apomucin was estimated as 58 000; then, this figure

was used to backcalculate the molecular weight of ovine submaxillary mucin and one came up with a value of 154 000 [45]. This study suggested that the mucins had a strong tendency to self- associate in 0.5 M NaCI.

Now, assuming 650 amino acids for the apomucin, and given a 0.364 nm extension per amino acid residue for an extended model, one can calculate a peptide length of 236 nm which is in good agreement with the maximum length observed in electron microscopy [441 . The smal- ler molecules may be interpreted as being less extended mucin molecules.

Recently, porcine submaxillary mucin has been almost completely deglycosylated with a mixture of glycosidases to give apomucin with a molecular weight of 96 500: this corresponds to a fully glycosylated mucin subunit with a molecular weight of about 275 000 [46].

Bhavanandan and Hegarty have also prepared an antiserum against deglycosylated bovine submaxillary mucins and used this immuneserum to identify the primary translation product of this mucin: it is a 60 kDa protein [47] and this is in excellent agreement with the data found for ovine submaxillary mucin.

In our laboratory, we have used a rather similar approach to identify the peptide precursors

Fig. 1. Electron micrographs of tungsten replicas of a) pig gastric mucins (128 000 x) and b) human tracheobronchial mucins (128 000 x). The pig gastric mucins were prepared by Dr. A. Allen (Newcastle upon Tyne) and the micrographs provided by the courtesy of Dr. H.S. Slayter from Boston (unpublished results).


Table !!. Size of mucin filaments observed by electron microscopy.

Mucin nm Ref.

Ovine submaxillary (OSM) Pig gastric (PGM) Ovarian cyst Human cervical Human respiratory

80-250 [44] 200-4000 (~90%, < 2000 nm) [43]

0-1200 [391 200-3500 (~90%, < 2000 nm) [43] 200-1400 [15, 42] 550-1100 [401 300-2500 (~90%, < 1500 nm) [41]

0-3000 (~80%, < 2000 rim) [43]

of human bronchial mucins. A series of peptides in the range of 200 to 400 kDa have been identified. If we compare these results with the length distribution of the respiratory mucin filaments observed by electron microscopy, one can inter- pret the EM results as a distribution containing several more or less extended mucins, having different Mrs [42]. However, these results are apparently at variance with those of Woodward et al., who found that the apoproteins obtained by chemical deglycosylation of canine or human bronchial mucin glycoproteins had a molecular weight about 100 kDa [48].

Properties of the mucin, peptide

The first chemical evidence of the heterogeneity of mucin peptides was produced in 1973 by De.gand et al., who observed differences in the amino acid compositions of different mucins contained in bronchogenic cysts [49]. Differen- ces in both amino acid and carbohydrate compositions have also been observed for colonic and small intestine mucins [50, 51] as well as for mucins contained in sinus mucoceles [52].

The concept of highly glycosylated regions and naked regions [18, 20] is widely accepted, although one does not know the extent and the localization of the naked regions.

Many studies have been devoted to the action of disulfide bond-breaking agents on mucins. The effects of these agents vary with the mucin preparations. In some mucin preparations, re-

ducing agents produce a decrease in Mr [20], while in other mucins there is apparently no effect on Mr [53]. In gastrointestinal mucins, a 'link protein', which may be attached to mucin subunits via disulfide bridges, is released together with mucin subunits [20]: the Mr of this link protein is 70 000 in the case of pig gastric mucin [20, 54], 118 000 in the case of human small intestine mucin [55]. Treatment of canine tracheobronchial mucin with disulfide-reducing or oxidizing agents results in the release of a 60 kDa component [56].

There are several possible interpretations or hypotheses for the action of disulfide bond-break- mg agents: 1) the mucins sensitive to reducing agents are multimers of subunits linked by disulfide bridges; 2) then, considering the EM pic- tures, the disulfide-linked subunits have to be linked end to end as suggested by Carlstedt et al.[2]; 3) reducing agents may activate proteolytic enzymes thought to be closely associated with mucins and which cleave the mucin peptide [57]; as a matter of fact, it is possible to inhibit the effects of reducing agents on respiratory mucins by adding mixtures of protease inhibitors prior to reduction [57]; 4) intramolecular disulfide bridges of mucins, or of closely associated peptides (possibly lectins), might maintain conformations that favor non-covalent associa- tions of mucin molecules.

Mucins may have hydrophobic properties: lipids have been reported to be non-convalently associated to respiratory [15, 56, 58], intestinal [59] and salivary mucins [60]. It is possible that these properties are dye to the naked regions:

The complexity of mucins 1475

for instance, gall bladder mucins have hydrophobic properties that are due to the naked regions, since they are lost after proteolysis of the mucin [61].

There have been reports suggesting the covalent attachment of fatty acid, probably to the naked regions of mucins from the digestive tract [62], and acyl-transferase activities have been detected [63, 64]. However, these findings have not been confirmed by other laboratories [65].

In summary of this first part, the general pic- ture that emerges for mucins is that of monomo- lecular filaments having a broad range of length. The question of disulfide-bound mucin subunits is still a matter of debate, although we feel that, in the case of human respiratory mucins, the effects of reducing agents may be explained without involving intermolecular disulfide bonds.

Mucin carbohydrate chains

Structure elucidation o f mucin carbohydrate chains

The carbohydrate moiety may be very simple and made of single N-acetylgalactosamine residues or N-acetylneuraminyl a 2-6 N-acetylgalactosamine, as in ovine submaxillary mucin [66]; it may be also very complex, as in blood group substances [67] or in human respiratory mucins Iraqi .

There have been several phases in the history of the structural elucidation of mucin carbohydrates chains. In the sixties, using mostly partial acid or alkaline degradation, Morgan, Watkins and coworkers, and Kabat and coworkers, identified some very important peripheral structures (the blood groups ABO, the Lewis a and Lewis b structures, the type 1 and type 2 chains ) [67, 69, 70].

Reductive /3-elimination was introduced by Carlson in 1968 [71]. This method, which allows the removal (not always complete) of entile reduced carbohydrate chains, is still the best method available.

During the seventies, the carbohydrate complexity of some mucin preparations was progres- sively realized but major progress was made within the last ten years with the application of HPLC to the isolation of mucin oligosaccharides [72-75] and also with the development of high resolution proton NMR spectroscopy. In this

field, one should acknowledge the prominent roles played by H. Vliegenthart and H. Van Hal- beck [76, 77].

Hounseli and Feizi have proposed that the carbohydrate chains of mucins be analyzed as three regions: the core close to the peptide, the backbone and the peripheral regions [78]. A few hundred carbohydrate structures from different mucins have been established and some general features of these carbohydrate chains are sum- marized in Fig. 2 -5 .

The only structural element shared by all mucin carbohydrate chains is the terminal N- acetylgalactosamine linked to the peptide.

Oligosaccharides cores are formed by various additions to this N-acetylgalactosamine residue linked to the peptide. They have been numbered according to the sequence of their discovery (Fig. 2): core 1, Gal/3 1-3 GalNac, was the first core to be described; it is very common [2, 3, 71]; in core 3, an N-acetylglucosamine residue is linked /3 1-3 to N-acetylgalactosamine [79, 80] and the glycosyltransferase responsible for this synthesis has been characterized [81]; cores 2 and 4 result from addition of N-acetylglucosamine (linked B 1-6) to the N-acetylgalactosamine residue, and the glycosyltransferase responsible for this synthesis has been detected in several tissue [82, 83]; core 5 has been recently described in salivary gland mucin glycoproteins of the Chinese Swiftlet [84]; core 6, where the N-acetylgalactosamine residue is substituted by two galactose residues, has been observed by the S!omianys and coworkers in human gastric mucin [85]; an additional structure (indicated by a question mark in Fig. 2) may be a degradation product which has been observed in meconium [86].

In many mucins, the N-acetylgalactosamine ~esidue of the carbohydrate-peptide linkage may be sialylated on position 6 either in disaccharide chains [66] or in longer chains [71.87. 881.

In the short chains of some mucins, the galactose residue of core 1 or 2, instead of being substituted by disaccharide units forming the backbone, is directly sialylated (on position 3) [84, 89-91] or sulfated (on position 6) [92] or is terminated by blood group A, B or H determinants [71, 93].

The backbones of longer mucin oligosaccharides contain different disaccharide units, either the type 1 chain (galactose/3 1-3 N-acetylglucosamine) or the type 2 chain (galactose/3 1-4 N- acetylglucosamine). They start from any of the galactose or N-acetylglucosamine of cores 1, 2,


C O R E S

't u GalNAc ~-O-Thr (or

3 /

Gal I$1 /

Ser)

GIcNAc It 1

GalNAc qz-O-Thr (or Ser)

3 GalNAc q~t-o--r hr 3

GIcNAc 1~ 1 /

\ "t. 6 GalNAc c~-O-Thr (or Ser)

(or Ser)

GIcNAc I~ 1

2 GalNAc tL-O-Thr (or Ser) 3

/ Gal I~ 1

GIcNAc 1~ 1X

6 4 3

/ GIcNAc l$1

GalNAc (z-O-Thr (or Ser)

Gal II 1 \

6 6GalNAc ~.-O-Thr (or Ser) 3

/ Gal I~ 1

5 GalNAc ~-O-Thr (or Ser) 3

/ Ga!NAc ,,. !

Fig. 2. The different types of cores found in the carbohydrate chains of mucins. The core indentified by a question mark might represent a degradation product.

3 or 4. One can find either linear or branched structures: the linear structures may contain only type 2 chains, corresponding to the i antigen [87, 94, 95], or only type 1 chains [96], or be 'hybrids' having both type 1 and type 2 structures [90, 96, 97] (Fig. 3); the branched structures either contain only type 2 chains corresponding to I antig~,n [94, 95, 97, 98] or mixed structures where it seems, so far, that the type 1 chains are always linked/3 1-3 to a type 2 chain or to an N- acetylgalactosamine residue [95, 96, 99] (Fig. 3).

Terminal type 1 chains may be fucosylated either on the galactose residue (H antigen), or on the N-aeetylglucosamine residue (Le a), or on both (Le b). The 3 position of galaetose may be substituted by N-acetylgalactosamine or galactose according to the blood group status

[67, 69, 70] (Fig. 4). In some mucins, these teP minal type 1 chains may be substituted on the 6 position of galactose by sulfate [92] or by N-acetyl- neuraminic acid [90].

As far as the terminal type 2 chains are con- cerned, more substitutions have been observed in different mucins (Fig. 5): suIfate can be attached to the 6 position of galactose [92]; sialic acid can be linked to the 3 or the 6 position of galactose [87, 95, 100[; again, galactose may be substituted in position 2 by a fucose residue, giving a determinant to which galactose or N-acetylgalactosamine may be added to the 3 position of galactose according to the blood group ABH status [67, 69, 70]; in some mucin preparations, a fucose residue may be linked a 1-3 to these type 2 chains and this is the SSEA-1 or X determinant

The complexity of mucins

BACKBONES

1477

Type 1

Ga1131- 3 GIcNAc

Type 2

[GaI131-4GIcNAc [

i ag 31 Ga, ,-4 G,c.Ac ]

/

I a, 1-4 G,c.Ac "[/"

Ga113 1-3 GIcNAc 13 1

3

/ iGa113 ~-4 GIcN, AC. 13 1 I

P~=I fl t_"1 P-I~,J* ' .~ 13 1 /

Gal f} 1-3 GIcNAc 13 1 3

I Ga1131-4 GIcNAc 13 1 L ~ | ag

" 6

31Ga, f}1-4 GlcNAc 13,,1 I

/

i. Ga1131-4 GIcNAc 13 1 (

IGal 13 1-4 GIcNAc I} 1 L /

N 6

3[Gal8 1-4GIcNAc I}'1 I

/ Ga! 8 !-3 G!cNAc 8 1

Fig. 3. Examples of 'backbones' found in the carbohydrate chains of rtlucins. The backbones are made of disaeeharide units (either type 1 or type 2 chains).

TERMINAL TYPE 1 CHAIN

Sulfate % . NeuAc (x 2 . , ~

6

3

A GalNAc (~ 1 " ~ /

B Gal (~ 1

Gal I] 1 - 3 G I c N A c I]1 - - 2 4

Fuc{xl Fucks1

H Le a

M~ , J

Le b Fig. 4. Possible sites of substitution on terminal type 1 chains.

[101]; this X determinant can be substituted by sialic acid (sialylated X) or fucosylated (Y determinant) [102, 103].

Less common structures have been described which raises the question of their tissue specifi- city (Fig. 5): unusual Gal fl 1-4 or G a l a 1-4 Gal /3 1-4 structures have been observed in Colloca- lia mucins [84]; the longer one is known to occur on £1ycolipid with blood group P1 activity; an N- acetylglucosamine linked t~ 1-4 to the galactose has been observed in duodenal mucin [104,105]; an N-acetylglucosamine l inked/3 1-3 has been observed in human [91] and rat [88] colonic mucins and in rat sublingual mucin, sometimes with an additional sialic acid [87]; finally, the Cad and Sd a determinants have been found in urinary mucin by Cartron etal. [106] and in monkey cervical mucin by Nasir-Ud-Din et al. [93].


TERMINAL TYPE 2 CHAIN

NeuAc a 2

S u l f a t e , ~

Gal {= 1-4 Gal [ 3 1 ~ 6

.4 1Ga1131-4 GIcNAcI3"}"

B Gal o: 1 i /

(NeuAc ~2)- 4 G I c N A c 1 3 1 / Fuc~l Fuc~l

H X

',,.. J

Y

Fig. 5. Possible sites of substitution on terminal type 2 chains.

In some mucin preparations, such as ovine or pig submaxillary mucins, the variety of carbohydrate chains is limited [66, 71]; in other preparations, colonic [95, 97], gastric [94], cervical [91, 93], ovarian [67, 96, 99] or respiratory mucins [68, 89, 90, 92, 98, 100, 103, 107, 108], there is a wide diversity of chains.

For instance, in collaboration with H. Vlie- genthart's group, we recently identified 60 different oligosaccharides having core 1, 2, 3 or 4 from the respiratory mucins of a single patient suffering frGm bronchiectasis and, for that speci- fic patient, the existence of several hundred different carbohydrate chains is highly problable [103, 1091.

Questions raised by the diversity of mucin carbohydrate chains

Our knowledge concerning the different glyco- syltransferases involved in the biosynthesis of mucins is still limited and will not be discussed here (for a general review, see [110]). However, many questions arise from the diversity of carbohydrate chains in mucins.

Why are some mucins more diverse in respect to carbohydrate chains than others ~' Using monoclonal antibodies on colonic mucins sepa- rated by ion-exchange chromatography, Podol- sky et al. have shown different mucin entities, each bearing a limited number of carbohydrate epitopes [111]. Therefore, the diversity of carbohydrate chains in molecules secreted by colonic

mucosae may correspond to a m;xture of carbt,- hydrate chains attached to different peptides.

Can this diversity be due to degradation? This may be the case for colonic mucins, for example [112-116].

Is the glycosylation process related to the mucin peptide and, if so, to what extent? For instance, is it possible that all the oligosaccharide cores found in a given mucin preparation (for example, respiratory mucins) coexist on the same peptide, or may appear at different times on the same peptide? Is there a synthesis of the different mucin peptides in a single cell, or in different cells, each of them leading to some speci- ficity of the glycosylation?

In some instances, there is evidence for existence of different mucin peptides with different carbohydrate chains. This is the case already mentioned for mucins from bronchogcnic cysts [49], from mucoeeles [52] and also for intestinal mucins [50, 51].

Do the different glandular cells of a mucosa synthesize the same mucin molecules? At the present time, some histological data tend to sug- gest that, in the duodenal mucosae, different mucous cells contain diffe,,'ent glycoconjugates, possibly different mucins [117, 118]. These results have been obtained by using various monoclonal antibodies or lectins. The ABH antigenic determinants expressed in the different glandular cells of human pyloric and duodenal . . . . . . . . . . . L . . . . . . . . . . . . . . . . . . . 17] I i lUI~L~d I l l d y U~ q U l t t ~ l l t~ t t~ lOgt~i l t~UU~ [1 .

There is also evidence that one single mucous cell may contain different types of mucins. At the electron microscopic level, the granules observed in individual respiratory mucous cells after staining with the periodic - thiosemicarbo- hydrazide -- silver proteinate may be quite het- erogeneous [119, 120]. Whether or not such pat- terns correspond to products at different stages of maturation or, more probably, to different mucins remains to be elucidated. Human colonic goblet cells also contain several types of mucins revealed by monoclonal antibodies [121].

One may also wonder if the complexity of carbohydrate chains is related to function at any given anatomical site. Right now, the prevalent hypothesis is that the diversity of mucin carbohydrate chains plays a role in the attachment of pathogenic strains to be eliminated for instance from the lung or from the oral cavity [122]. In the gastrointestinal tract, the attachment of sym- biotic strains might protect the mucosae from infection by pathogenic strains.

In this regard, Table IIl lists a few carbohy-

The complexity o f mucins

Table i!1. Examples of carbohydrate structures recognized by microorganisms.

1479

Carbohydrate Organism Ref.

NeuAc a 2-3 Gal/3 1-3 GalNAc Streptococcus sanguis [ 123]

NeuAc a 2-3 Gal/3 1 -4 GIcNAc Gal B 1-4 GlcNAc/3 1-3 Gai/3 1-4 Glc Gal/3 1-3 GIcNAc/31-3 Gal/3 1-4 Glc Gal ot 1-4 Gal (glycolipid) Gal/3 1-3 GalNAc (glycolipid) NeuAc a 2-3 Gal (glycolipid)

NeuAc a 2-6 Gal

Streptococcus mitis [ 123] E. co!i (type S. fimbriae) [124] Mycoplasma pneumoniae [125] Streptococcus pneumoniae [ 126] Streptococcus pneumoniae [ 126] E. coli (type P fimbriae) [127] Actinomyces naeslundi [128] Influenza virus A / PR / 8 / 34 [129] (H1N1) Influenza virus A / Japan / 305 / 57 [129] (H3N2)

drate structures [123-129] that have been recognized as possible sites of attachment for various bacteria or viruses. Similar structures can be found on different mucins and it is tempt- ing to speculate that they play a role in adhesion of bacteria or viruses and protect the underlying epithelia.

As a matter of fact, human salivary mucins are known to mhibit influenza virus hemagglutina- tion [130] and tracheobronchial mucins have receptors for Pseudomonas aeruginosa [ 131 ].

. . . . . . . • . , ~ , o . u , , , , . , . ~ , ~ ,,uw c .uugn evidence to support the thread-like model for mucins. The diversity of carbohydrate chains that is found in certain mucins is amazing. The evidence for different mucin peptides in some mucosae seems to be quite clear but it is too early to evaluate the importance of this diversity in the different glands or mucosae that produce mucins. A lot of questions have been raised by these observations and there is no doubt that some of these will be answered in the near future, by the cloning of tht. mucin genes.

Acknowledgments

We are grateful to I. Skrzypczak and F. Deburck for typing the manuscript.

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