Biology Reference
In-Depth Information
10
Molecular reconstruction
of ancient genes/proteins
10.1 Introduction
Proteins from extinct organisms can be studied by the analysis of DNA recovered
from preserved organic specimens. This approach, however, requires biological
material that has been completely protected from the oxidative effects of oxygen
and water (Audic and Beraud-Colomb, 1997; DeSalle
et al
., 1992; Pääbo, 1993;
reviewed by Li, 1997) and significant doubts may remain as to the authenticity of
the DNA recovered (Austin
et al
., 1997; Stoneking, 1995). Alternatively, an
attempt can be made to reconstruct the amino acid sequence of an ancient protein
from the sequences of its extant descendants (Malcolm
et al
., 1990; Shih
et al
.,
1993; Stackhouse
et al
., 1990). This methodology relies upon the principle of max-
imum parsimony (i.e. the assumption that extant proteins have evolved from that
of an extinct ancestor by the smallest number of mutational changes). DNA
sequence data can also be treated in the same way (Hillis
et al
., 1993), one example
being the investigation of ribosomal DNA phylogeny (Friedrich and Tautz, 1995).
Jermann
et al
. (1995) examined the evolutionary history of artiodactyl RNase A,
a pancreatic digestive enzyme, derived the ancestral enzymes phylogenetically
and reconstructed them by site-directed mutagenesis. The kinetic properties of
the reconstructed enzymes were found to be similar to those of extant RNases but
less stable to thermal denaturation, more susceptible to proteolysis and five-fold
more active toward double-stranded RNA. A Gly38
Asp substitution, which
occurred ~40 Myrs ago around the time when foregut rumination evolved, was
found to be associated with the reduction in activity toward double-stranded
RNA.
A similar analysis has been attempted with the chymases (mast cell proteases)
which hydrolyse angiotensin I to generate angiotensin II, a potent vasoconstrictor
hormone. Primate
-chymases are highly specific and only hydrolyse the Phe8-
His9 peptide bond. By contrast, rat
-chymase is less specific and further hydrol-
yses angiotensin I by cleavage of the Tyr4-Ile5 bond. Chandrasekharan
et al
.
(1996) determined the phylogeny of four mammalian
-chymases and six mam-
malian
-chymases and reconstructed the putative ancestral enzyme by chemical
synthesis. This enzyme proved capable of cleaving angiotensin I at the Phe8-His9
