Spider Silk - The Functional Fold: Amyloid Structures in Nature - page 75

Table 5.1 Selection of silk structures produced by insects and spiders

Protein
structure Protein characteristics Silk origin

α-helical Low glyceine content
High content of acidic residues

Insects
Spiders (when ejected or
extruded instead of spinning)

Converts to parallel-β

Parallel-β Classified in 6 Warwicker groups,
characterized by increasing side
chain length and decreasing glyceine
content
Polypeptide chain oriented parallel
to fiber axis

Insects

Spiders

Cross-β High serine content

Polypeptide chain oriented
perpendicular to fiber axis

Insects
Spiders ( , formed by

a recombinant spider silk
protein)

in vitro

1,2

Converts to parallel-β structure
under stress

Most spider silks belong to the class of parallel-β silks, but upon
ejection or extrusion of spider silk proteins also α-helical structures
can be adopted. It has to be noted that additional structural subclasses
exist, which are not discussed in context of this review.
The α-helical silks are mainly produced by insects like bees and
fleas that do not possess glands that are entirely dedicated to silk
production. Such silks are used for reinforcement, nets, pupation
cases, or cocoons. Although no spider is known to produce α-helical
silks, it is assumed that spitting spiders eject or spit silk with
α-helical structure, for example. It has been found that α-helical silk
can be converted into parallel-β structure under shear stress.

5,6
Silks with parallel-β-pleated sheet structure showing either
parallel or antiparallel orientation of the strands within the sheets
are produced by both insects and spiders. Warwicker classified
parallel-β-silks into six sub groups differing in length of the side
chains and in their glycine content.

7
A third type of silk structure is a cross-β-pleated sheet which
is produced by insects like the lacewing

. Cross-β
silk is used by lacewing larvae for cocoon production as well by
adults for attaching eggs to the lower side of leaves. This type of
structure resembles that of amyloid fibrils but it can be converted
into parallel-β silk upon stretching.

Chrysopa flava

5,6