Spider Silk - The Functional Fold: Amyloid Structures in Nature - page 82

5.2.2 Assembly of Silk Proteins

Very little is known about the assembly of cross-β silks. They are
produced by some insect larvae in Malpighian tubules and, addi-
tionally, in colleterial glands of adult green lacewings (

5,57

C. flava ).

The colleterial glands are sac shaped and composed of columnar
secretory cells containing an apical end apparatus reflecting
infoldings of the plasma membrane. Analysis of sectioned material
revealed the presence of sheet-like micelles which are several
hundred angstrom wide and 25 Å thick, matching the dimensions of
the ribbon-like structure calculated from XRD. Malpighian tubules

are a type of excretory and osmoregulatory system and represent
an example for glands not primarily dedicated to silk production. As
the system is usually applied to absorb water and contains active ion
transporters, one might speculate that the later are also involved in
silk assembly.
Parallel-β silks are much more thoroughly analysed and several
aspects of their assembly process have been unravelled. Two of the
best characterized parallel-β silks are the cocoon silk from

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B. mori

and spider dragline silks from species. The

underlying proteins are produced in salivary glands in the case of
the silkworm

Nephila and Araneus

B. mori and the major ampullate glands in case of the

spiders. The assembly processes of both silk types are similar

regarding the involved factors, therefore, for simplification, we will
only focus on the assembly of spider dragline silk in more detail.

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5.2.2.1 Crucial factors triggering spider silk protein assembly
After secretion from specialized, tall columnar endothelial cells into
the lumen of the ampulla, dragline silk proteins, also called major
ampullate spidroins, are stored mainly intrinsically unstructured or
at least only partially structured at very high concentrations (up to
50% w/v) in the liquid dope (Fig. 5.2).

4

To form the final thread, they
have to undergo a liquid — solid phase transition which is accom-
panied by the formation of β-sheet structures (as described earlier)
stabilized by intra-and inter-chain hydrogen bonds. Factors

involved in the extension, alignment, and dense packing of the
molecules include physical forces like increasing elongational flow
in combination with biochemical changes in the spinning duct.

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46,59

These are in detail an increase in the concentration of kosmotropic
phosphate ions, while chloride ions are assumed to be extracted. The