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with the ~6 nm thickness of individual spore coat layers. In the case of
globular proteins, lateral lattice parameters typically do not differ to such
an extent from the height of growth layers, which is relected in relatively
small differences between lateral and perpendicular crystallographic unit
cell parameters.
27
Thus, the proteins forming the spore coat layers are likely
not globular, but rather may be stretched peptides “standing upright” in the
layers. This construction, which is
found in fat crystals,
28
results in a crystal
class with relatively strong, hydrophobic interaction forces between the long
neighbouring units (here peptides) and weak interaction forces between
the different crystalline layers. Such a crystal type, with tightly packed,
strongly interacting longitudinal peptides within a layer, would help explain
the toughness associated with bacterial spore coats.
1-3,13
It may also explain
why spore coat proteins are dificult to dissolve,
1,13,29
as this type of packing
involves hydrophobic interactions and hence a high proportion of hydrophobic
amino acids.
In addition to enabling the nucleation and growth of new coat layers
during sporulation, the screw dislocations also pin several of these layers
together, thereby making the spore coat an interconnected, cohesive entity,
rather than a set of separate layers loosely deposited on top of each other.
This, combined with the strong in-layer bonds, and possible cross-linking
between the coat proteins, likely contributes to the resilient nature of the
spore coat.
In biology, crystallization is most often associated with biomineralization,
where protein-directed crystallization leads to calcious bone
30
and shell
formation.
31,32
Screw dislocations and ensuing spiral growth have been
observed for shell formation.
33,34
High-resolution scanning electron probe X-
ray microanalysis
35,36
and nanometre-scale secondary ion mass spectrometry
37
studies have demonstrated that the proteinaceous coat of several bacterial
spore species is essentially devoid of divalent mineral cations such as
calcium, magnesium and manganese. This indicates that
spores
could present the irst case of non-mineral dislocation growth patterns being
revealed for a biological organism.
C. novyi-NT
4.1.3 Formulaon-Specific Spore Coat Assembly
The implication of observed crystalline nature of
spore coat layers for bacterial spore coat assembly is that, while the
proteineous building blocks are produced via biochemical pathways directed
by various enzymes and factors,
Bacillus
and
Clostridium
the actual construction of these building
blocks into spore coat layers is a self-assembly crystallization process.
Similarly, the striking differences in native rodlet motifs seen in
1
B. atrophaeus
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