in the attachment of chondrocytes to collagen. A family
of cell surface adhesion receptor proteins known as
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,
Red Blood Cell Membrane and
Membrane Skeleton Proteins
A major integral membrane glycoprotein of human red
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
S ia -a (2
6)-G al-/?(1 -> 3 U
J ^ G a lN A c-O -S er/T h r
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
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
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.]