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as well as the duplication of a primordial gene cluster. The tandem duplication of the
ancestral
subunit gene appears to have occurred before the duplication and translo-
cation of at least one of the original gene clusters.
G protein
subunit genes. G proteins are ubiquitous in eukaryotes and serve to
mediate signal transduction via appropriate receptors (see Section 4.2.3, G-pro-
tein-coupled receptor genes ) by coupling extracellular signals to intracellular effec-
tors such as adenylyl cyclase, phospholipases and ion channels. More than a
dozen G protein
subunit genes have been identified in the human genome
( Figure 4.10 ) and their evolutionary history appears to stretch back over 1500
Myrs (Wilkie et al ., 1992). Whilst some members of the human gene family are
chromosomally linked [e.g. GNAI2 and GNAT1 (3p21), GNAI3 and GNAT2
(1p13), GNA11 and GNA15 (19p13)], the remainder are scattered around the
genome. This organization is potential explicable by a combination of successive
genome duplications, tandem duplication, and duplication/translocation events
(Wilkie et al ., 1992).
Globin genes. Proteins homologous to the globins are ubiquitous in eukaryotes
and even have counterparts among the prokaryotes (Hardison, 1998; Riggs, 1991).
The myoglobin ( MB ; 22q11.2-q13) gene and the ancestor of the extant globin
genes are thought to have arisen from a common ancestral gene encoding a heme-
containing protein ~700 Myrs ago before the advent of the vertebrates
(Czelusniak et al ., 1982; Suzuki and Imai, 1998). The
-globin genes
diverged from each other about 500 Myrs ago and at some stage became chromo-
somally separated (Efstratiadis et al ., 1980). The
- and
-globin cluster subsequently
evolved by a process of successive duplication and divergence ( Figure 4.11 ): the
/
gene divergence occurring about 400 Myrs ago (Czelusniak et al ., 1982;
Proudfoot et al ., 1982) and the
/
gene divergence about 260 Myrs ago (Hsu et al .,
1988). The
2-globin genes arose from a further duplication event
between 50 Myrs and 60 Myrs ago whilst the
1- and
1 pseudogene was inactivated
~45 Myrs ago (Proudfoot and Maniatis, 1980; Figure 4.11 ). The 40 kb human
GNAS1 (20q13.2)
GNAL (18p11)
GNAI2 (3p21)
GNAI3 (1p13)
GNAI1 (7q21)
GNAO1 (16q13)
GNAT1 (3p21)
GNAT2 (1p13)
GNAZ (22q11)
GNAQ (9q21)
GNA11 (19p13)
GNA14 (?)
GNA15 (19p13)
GNA12 (?)
GNA13 (?)
40
50
60
70
80
90
100
Figure 4.10. Human G protein
Amino acid identity (
)
subunit multigene family.
 
 
 
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