Contractions and expansions in gene size and number - Human Gene Evolution - page 367

region of the human involucrin ( IVL ; 1q21) gene lies within a single exon and
encodes a 585 amino acid glutamine-rich protein (Eckert and Green, 1986). The
human IVL gene is extemely rich in CAG repeats (encoding glutamine) and
codons derived from it (GAG and CTG). Thus, CAG codons, and codons one sub-
stitution removed from CAG, comprise 64% of the total codon number. It is pos-
sible therefore that this gene is descended from a simple poly(CAG) sequence
which has been subsequently modified by nucleotide substitution (Tseng, 1997;
Figure 8.14 ).
The time of origin of the involucrin gene is unclear but the gene is present in a
wide variety of mammals and may be present in all terrestrial vertebrates. The
length of the involucrin molecule has continued to grow by successive addition of
repeats from prosimians (average 409 residues) through New World monkeys
(average 488 residues) and Old World monkeys (average 514 residues) to homi-
noids (average 632 residues). The coding region outside the segment of repeats
has by contrast changed very little in length. Since glutamine residues serve as
amine acceptors in the transglutaminase-catalyzed cross-linking, the primordial

ATG

P M

Anthropoids
Tarsioids
Non-primates
and prosimians

80 - 82
75
77 - 83

6
242
94 - 255

65 - 66
9
64 - 71

232 - 637
17
6-10

43 - 46
44
0-40

Figure 8.15. Coding regions of primate involucrin genes illustrating the relative locations
of repetitive regions P and M. Numbers given denote lengths (in codons) of segments of
repeats and non-repetitive regions (redrawn from Green and Djian, 1992).

Number of
altered
codons
per repeat
5
10
10
3
2
3
2
7
6
10
Mean 5.8

Repeat

Number
10
9
8
7
6
5
4
3
2
1

Ty p e
A
B
A
B
A
B
A
B
A
A

1
K
E
K
E
K
E
K
E
K
K

2
H
L
H
L
H
L
H
H
H
D

3
L
P
L
P
L
P
L
P
L
L

4
E
E
E
E
E
E
E
E
E
E

5
Q
Q
Q
Q
K
Q
Q
Q
Q
Q

6
Q
Q
E
Q
Q
Q
Q
Q
Q
Q

7
E
V
E
E
E
V
E
E
E
E

8
G
G
K
G
A
G
K
G
G
G

9
Q
Q
Q
Q
Q
Q
Q
Q
Q
Q

10
L
P
L
L
L
P
L
L
L
L

Figure 8.16. Consensus amino acid sequences (from 12 anthropoid ape species) for each
of the 10 repeats of the early region of the M segment of involucrin. Alterations in one or
more species are boxed or framed. Solid lines indicate deletions whilst dashed lines
indicate amino acid substitutions. Over half of the amino acids have been deleted or
replaced in one or more species. The doubly underlined Q in repeat 5 is the preferred
residue for cross-linking in human involucrin (redrawn from Green and Djian, 1992).