Abstract:
Flagelliform spider silk is a highly extensible protein polymer produced by
orb weaver spiders with remarkable mechanical properties combining strength with
incredible extensibility. The flagelliform fibers are composed of a single protein, Flag,
and can stretch up to six times their original length. The core of the large 400 kDa Flag
protein is composed of three simple amino acid motifs. The GPGGX motif has been
previously shown to contribute the elasticity of the silk fibers but the source of the
strength of these fibers has been elusive.
The GGX and “spacer” core motifs were selected for the purpose of this study to
determine their specific contributions to the fiber mechanical properties and structure.
Four recombinant proteins were designed and genetically engineered to contain repeats of
the GGX; the GGX and GPGGX, the GGX and spacer; or the GGX, spacer and GPGGX
modules. The recombinant proteins, ranging in size from 55-65 kDa, were utilized as
molecular tools to investigate the mechanical property contributions of the individual
motifs to the silk fibers.
Structural studies of the recombinant silk fibers provided new insight into motif
structural contributions of the native flagelliform Flag protein. Raman spectroscopic
analyses correlate with an increase in β-turn structure with increased spacer motifs in the
recombinant proteins. Wide Angle X-ray diffraction studies provide evidence of
increased molecular structural orientation correlating with the inclusion of the spacer
motif. The GGX and GPGGX fibers produce amorphous X-ray diffraction data. These
results suggest that the spacer motif is a major contributor to the strength of native
flagelliform silk. These recombinant proteins provide a platform for additional studies
that will contribute to the understanding of the structure/function correlation that produce
the material mechanical properties of flagelliform silk.