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outside of the barrel via a disulphide bond. Two disulphide bonds
cross-link the two strands within each of the two
-hairpins and
the fourth disulphide bond connects the N-terminal loop to the core
β
β
-barrel.
Similarly, the core structure of EAS is centred on two interlocking
β
β
-hairpins that form a four-stranded
-barrel. However, in place of
the
-helix found in the HFBI/II crystal structures, there is a short
two-stranded antiparallel
α
β
-sheet. The other significant difference is
that EAS has two disordered loops that extend from the barrel. The
longer of the two lies between the third and fourth cysteines. This is a
region of high variation between Class I hydrophobins, both in terms
of sequence and length. This disordered loop is absent from Class II
hydrophobins. The structure of EAS demonstrates how the inter-
cysteine variation observed in hydrophobins can be accommodated
on the periphery of the conserved hydrophobin barrel and it is likely
that this is the case for all Class I proteins, given they all share the
same pattern of cysteine residues.
The three-dimensional structures of these hydrophobins reveal
the structural basis for their surface activity and ability to form
amphipathic monolayers. Both Class I and Class II molecules display
large hydrophobic patches on the surface, which account for at least
one-fifth of their total surface area. In EAS, there are relatively few
charged residues and these are clustered on a single face of the
protein, giving rise to a molecule with distinct, opposing hydrophilic
and hydrophobic faces (Fig. 3.6d). The charged residues in HFBI
and HFBII are more evenly distributed over the surface so that the
molecules do not display such a striking amphipathic character.
However, the presence of the hydrophobic regions on the surface of
these proteins is thought to drive the orientation of both classes of
hydrophobin at a hydrophobic/hydrophilic interface and thus, the
assembly of the monolayer.
A comparison of the monomeric structures of the Class I and
Class II members does not obviously identify features which can
explain the fundamental differences in the secondary structure
content, stability, and morphology of their assembled forms. Instead,
it is possible that the differences between the classes of hydrophobins
arise from variations in their conformational plasticity and dynamics,
and in their differing ability to access structural intermediates which
are prone to intermolecular aggregation through hydrogen bonding.
In the amyloid field, for example, amyloidogenic variants of proteins
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