The nonrepetitive and highly conserved C-terminal region
was also identified in all described N. cruentata spidroins.
Alignment of the C-terminal amino acid sequence described for
these spidroins with those of the known fibroin genes showed
great sequence conservation, with the more highly conserved
sequences among ortholog gene groups than among paralogous
genes. Most spider silk proteins share 30% identity among their
C-terminal, w ith flagelliform spidroins the most highly
diverged. However all sequences share a particular conserved
region at the QALLE amino acid sequence even between orb-
web and non-orb-web weavers spiders. This region correspon ds
to the region with higher hydrophobicity in the C-terminal, and
secondary structure predictions suggest that this QALLE region
is also responsible for the formati on of α-helices (Spooner et al.,
2005; Challis et al., 2006). Since the C-terminal sequence is
much more conserved than the repetitive region among different
spiders' species, it is suggested that it might play an important
role with certain amino acid motifs being preserved by selection
against mutations that could disrupt the C-termina l function
during evolution. Several functions have been attributed to the
C-terminal from silk proteins. It might be responsible for the
formation of irregular sized micelles in the spinning dope in
order to prevent premature fiber formation (Jin and Kaplan,
2003) or have a function in correct protein folding since it has
been demonstrated that the C-terminal is retained in the fiber
(Sponner et al., 2004; Ittah et al., 2006).
Because of its high sequence conservation, C-terminal region
alignment was used for phylogenetic analysis. The result shows
that different ortholog genes cluster together rather than by
species, agreeing with previously phylogenetic analysis of the
spidroin gene family (Gar b and Hayashi, 2005; Challis et al.,
2006; Garb et al., 2006); all identified N. cruentata spidroins
grouped together in their specific gene order. The observation
that silk proteins cluster according to their type suggests that
their evolution may be due to a divergent evolution involving a
common ancestor, rather than by a model of concerted evolution,
where one would expect genes to cluster within species and not
within the same gene order (Challis et al., 2006; Garb et al.,
2006). Based on fossil evidence, a possible common ancestor
could be the extinct Macryphants, dating from the lower
cretaceous at least 136 million years ago (Selden, 1989).
In summary, we have studied different silk spidroins from the
Brazilian spider N. cruentata. We were able to identify the
protein sequences from sil ks responsible for orb-web building
and prey capture (MaSp, MiSp and Flag), as well as for the
construction of the eggcase (TuSp). Throughout our study it was
possible to demonstrate a high degree of similarity between
these sequences and sequences from other spider species. It was
also observed similarities among sequences from different gene
groups with the exception of tubuliform spidroin that present a
complete different protein structure, and which play a distinct
function in the spiders' life (Hu et al., 2005a; Tian and Lewis,
2005). These results support the argument that mechanical
properties are correlated to their protein sequences (Hayashi
et al., 1999; Rising et al., 2005). Further mechanical and
structural study of N. cruentata spidroins shoul d also provide
more evidence for this correlation.
Acknowledgements
We would like to thank Dr Michael Hinman and Dr David
Perry (University of Wyoming) for reviewing this work and
Kelly Martins de Brito for technical assistance. This research
was funded by CNPq (Conselho Nacional de Desenvolvimento
Cientifico e Tecnologico-Brazil), grant: 486492/2006-0.
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