Chemistry Reference
In-Depth Information
20.6
Non - Metazoan CTLD s : From Viruses, Bacteria and Protozoa
To recapitulate what we have learnt so far: it had been thought that the primordial
CTLD had originated at the beginning of the Metazoan era and had a carbohydrate-
binding function. As shown in Figure 20.5 , pre - genome and early - genome studies
were consistent with this model showing that CTLDcps were abundant in Metazoa
but absent in non-Metazoa such as fungi (genome of the yeast
Saccharomyces
cerevisiae
). Notwithstanding these general fi ndings, many interesting examples of
non-Metazoan CTLDcps have been reported [2].
Many of these CTLDcps are from parasitic viruses and bacteria which are
involved in interactions with the animal host, often as mechanisms to defeat its
immune system. The CTLDs of the viral proteins (for example fowlpox, vaccinia
and African swine fever viruses) contain sequences similar to mammalian CTLDcps
[2]. Well-conserved CTLD sequences are also present in
Trypanosoma
, a parasitic
protozoan. The best-characterized bacterial group are toxins (pertussis toxin and
proaerolysin) and adhesion proteins (intimin from
Escherichia coli
and invasin
from
Yersinia pseudotuberculosis
). Details on bacterial toxins and adhesins are pre-
sented in Chapter 18. Their CTLDs cannot be identifi ed by sequence analysis and
their structures show a more compact fold [2, 4]. However, several CTLDs (see
Info Box 2 for examples) found in sequenced genomes from free- living bacteria
contain well - conserved
E
P
N
/W
N
D motifs suggesting they are Ca
2+
/sugar binding,
although some lack conserved cysteines.
How might these CTLDcps in bacteria and viruses have arisen? The most par-
simonious explanation of the presence of the viral, protozoan and bacterial CTLDs
homologous to those in Metazoans is horizontal gene transfer (that is they are
hijacked host proteins in viruses or otherwise acquired) [2]. The high sequence
and structural divergence of the CTLD of the parasitic bacterial proteins obscure
their origin. They may also have been acquired by horizontal gene transfer or,
Info Box 2
Examples of free-living bacteria with CTLDs found by genome analysis are:
Leeuwenhoekiella blandensis
, a marine fl avobacterium;
Synechococcus
sp. RS9917,
a marine photosynthetic cyanobacterium;
Marinomonas
sp. MED121, a marine
proteobacterium; and
Stigmatella aurantiaca
, a gliding, Gram-negative bacte-
rium. A particularly intriguing example is a putative CTLDcp deduced from the
genome sequence of a marine planctomycete
Pirellula
sp. This is the largest
protein in the genome (7716 residues) and it contains several CTLD, laminin
G and cadherin domains, all of which are domains almost exclusively found in
Metazoa (for further information on laminin G domains, see Chapter 16 ). What
could be the function of such a complex protein in a free-living species such
as
Pirellula
?
