section 10.3

Cell-Surface Glycoproteins

163

in the attachment of chondrocytes to collagen. A family

of cell surface adhesion receptor proteins known as

integrins

binds with fibronectin. Integrins are a fam-

ily of proteins containing «/3 heterodimers and pos-

sess receptors not only for fibronectin but for collagens,

laminin, fibrinogen, vitronectin, and integral membrane

protein of the immunoglobulin superfamily (Chapter 35).

A sequence of 3 amino acids (RGD) in fibronectin

is a recognition site for binding with integrin. Other

amino acid sequences that are present in proteins for

integrin recognition include KQAGDV, DGEA, EILDV,

and GPRP.

Red Blood Cell Membrane and

Membrane Skeleton Proteins

A major integral membrane glycoprotein of human red

blood cells,

glycophorin A,

has been characterized. It has

a molecular weight of 31,000. Each membrane contains

about 400,000 molecules of glycophorin, accounting for

about 1.5% of the weight of the membrane, while the car-

bohydrate portion constitutes about 40% of the weight.

The polypeptide consists of 131 amino acid residues

(Figure 10-11). The transmembrane protein has three dis-

tinct domains: a hydrophilic amino terminal end that

extends outside the membrane and contains all of the

oligosaccharide side chains; a hydrophobic middle region

buried in the lipid bilayer; and a hydrophilic region rich

in charged residues that protrudes into the cytosol. Gly-

cophorin A has 16 oligosaccharide units, of which 15

are linked by O-glycosidic bonds to serine or threonine

residues and one is linked by an N-glycosidic bond.

All of the O-linked oligosaccharides have the following

disialotetrasaccharide:

S ia -a (2

- *

6)-G al-/?(1 -> 3 U

J ^ G a lN A c-O -S er/T h r

Sia-«(2->6)^^

where Sia = sialic acid, Gal = galactose, GalNAc = N-

acetylgalactose, and Ser/Thr = serine/threonine.

The N-linked oligosaccharide contains mannose. All

of the oligosaccharide chains appear in the 50 residues

of the amino terminal domain, in which amino acid

residues 2^4 and 10-15 all carry an oligosaccharide

unit in O-glycosidic linkage. The middle hydropho-

bic domain is believed to have an «-helical structure.

Glycophorin contains antigenic determinants for blood

groups M and N (see below). The M and N anti-

genic determinants are present on glycophorin A and

glycophorin B, respectively. The former differs from

the latter at positions 1 and 5 of the amino terminal

end:

H

2

N-Ser-Ser-Thr-Thr-G

5

ly-

Glycophorin A

H

2

N-Leu-Ser-Thr-Thr-Glu-

Glycophorin B

Glycophorin A is present in type M, glycophorin B in

type N, and both glycophorins in type MN individuals.

Antibodies directed against M and N do not cross-react

and can distinguish sequence differences between the two

glycophorins. Although some antibody preparations may

FIGURE 10-11

Primary structure of glycophorin A. There are 15 O-linked and 1 N-linked carbohydrates attached to the protein.

[Adapted from L. W. Stryer,

B io ch em istry,

3rd ed. W. H. Freeman, New York, 1995.]