Consensus
motifs for sequences of both crystallizable and amorphous blocks in silks and natural structural
analogues of silks vary widely. To design novel silklike polypeptides,
an important question is therefore how the nature of either the crystallizable
or the amorphous block affects the self-assembly and resulting physical
properties of silklike polypeptides. We address herein the influence
of the amorphous block on the self-assembly of a silklike polypeptide
that was previously designed to encapsulate single DNA molecules into
rod-shaped viruslike particles. The polypeptide has a triblock architecture,
with a long N-terminal amorphous block, a crystallizable midblock,
and a C-terminal DNA-binding block. We compare the self-assembly behavior
of a triblock with a very hydrophilic collagen-like amorphous block
(GXaaYaa)132 to that of a triblock with a less hydrophilic
elastin-like amorphous block (GSGVP)80. The amorphous blocks
have similar lengths and both adopt a random coil structure in solution.
Nevertheless, atomic force microscopy revealed significant differences
in the self-assembly behavior of the triblocks. If collagen-like amorphous
blocks are used, there is a clear distinction between very short polypeptide-only
fibrils and much longer fibrils with encapsulated DNA. If elastin-like
amorphous blocks are used, DNA is still encapsulated, but the polypeptide-only
fibrils are now much longer and their size distribution partially
overlaps with that of the encapsulated DNA fibrils. We attribute the
difference to the more hydrophilic nature of the collagen-like amorphous
block, which more strongly opposes the growth of polypeptide-only
fibrils than the elastin-like amorphous blocks. Our work illustrates
that differences in the chemical nature of amorphous blocks can strongly
influence the self-assembly and hence the functionality of engineered
silklike polypeptides.