Biology Reference
In-Depth Information
Table 4.3.
Types of human integrin chain and the genes that encode them
Integrin chain
Gene symbol
Chromosomal location
1
ITGA1
5
2
ITGA2
5q23-q31
3
ITGA3
17
4
ITGA4
2q31-q32
5
ITGA5
12q11-q13
6
ITGA6
2
7
ITGA7
12q13
8
ITGA8
?
9
ITGA9
3p21.3
IIb
ITGA2B
17q21.3
E
ITGAE
?
L
ITGAL
16p11.2
M
ITGAM
16p11.2
V
ITGAV
2q31-q32
X
ITGAX
16p11-p13
D
ITGAD
16p11.2
1
ITGB1
10p11.2
2
ITGB2
21q22.3
3
ITGB3
17q21.3
4
ITGB4
17q11-qter
5
ITGB5
?
6
ITGB6
2
7
ITGB7
12q13.1
8
ITGB8
7p
the conserved central alpha-helical domains, the variable terminal glycine-rich
domains of keratins 1 and 10 (comprising short 4-10 amino acid segments
repeated 3-15 times) appear to have evolved by a series of tandem duplications
brought about by unequal crossing over (Klinge
et al
., 1987).
In the human genome, the genes encoding these keratin families have been
found to be tightly clustered; at 17q12-q21 for the acidic keratins (e.g.
KRT9
,
KRT10
,
KRT14
,
KRT15
,
KRT16
,
KRT17,
and
KRT19
), and chromosome 12 for
the basic keratins (e.g.
KRT1
,
KRT2A
,
KRT5
,
KRT6A
,
KRT6B,
and
KRT8
)
(Milisavljevic
et al
., 1996;
Table 4.4
). There is one exception to this rule:
KRT18
is
a type I keratin gene but is located on chromosome 12q11-q13. There is some evi-
dence for gene conversion between type I and type II keratin genes which could
have facilitated their coevolution (Klinge
et al
., 1987). Concerted gene duplica-
tions also provide evidence for coevolution of type I and type II keratin genes
(Blumenberg, 1988). Thus, in both families, the genes expressed in the embryo
duplicated and diverged first, followed by the genes expressed in various differ-
entiated cells. Further gene duplications gave rise to the hair keratin genes
(Blumenberg, 1988; Powell
et al
., 1992; Rogers
et al
., 1998;
Table 4.4
). This para-
llelism of gene duplication cannot be explained by gene conversion but as yet the
underlying mechanism which apparently allows duplications in one family to
influence duplications in the other is still unclear. Coevolution may have been
driven by the obligate heteropolymer status of the proteins. The tight regulation
of the coordinate expression of the type I and type II keratin genes implies that
unbalanced production is likely to be deleterious. It may therefore follow that the
