the threads, namely that they are elastic on the proximal
end and rigid at the distal end.
These correlations were supported by isolation of cDNAs
encoding Col-D and Col-P precursor proteins, PreCol-D and
PreCol-P, respectively (Table 1). The sequence of PreCol-D
revealed silk fibroin-like domains adjacent to the collagenlike
domains [34•]. Presumably the silk-like domains stiffen
the threads by formation of crystalline β sheet structures, as
found in natural silk fibers. The sequence of PreCol-P
revealed a collagen-like domain which was flanked by
sequences resembling elastin (i.e. contain a high percentages
of glycine, proline and bulky hydrophobic residues) [35••]. It
has been proposed that these elastin-like domains are
responsible for the flexibility of byssus threads at the proximal
end. While these investigations of byssus fiber are still in
their early stages, the breakthroughs obtained to date have
already revealed new intriguing strategies by which nature
assembles structural materials.