Unique molecular architecture of egg case silk protein in a spider, Nephila clavata

We describe a unique silk protein secreted from the cylindrical silk glands of the spider Nephila clavata. This silk is primarily composed of three proteins, whose transcripts of approximately 16.0, 14.5 and 13.0 kb are homologous to one another in two termini and repetitive units, as determined on Northern blotting. Its overall organization shows that it is similar to other characterized silk proteins, including in the mainly central repetitive region as well as the non-repetitive N-terminal (166 residues) and C-terminal (176 residues) parts. However, up to 90% of the protein consists of highly ordered repetitive structures that are not found in other silks. The repetitive region mainly consists of several types of complexes and remarkably conserved polypeptide repeats. The assembled repeat units (A1B1) contain a high proportion of Ala (30.41%), Ser (25.15%), and residues with hydrophobic side chains (22.22% for Gly, Leu, Ile, Val and Phe combined). The presence of Ser-rich and GVGAGASA motifs suggests the formation of a beta-sheet. The repetitive region is characterized by alternating arrays of hydrophobic and hydrophilic blocks. The results suggested that this egg case silk is an exceptional protein when compared with previously investigated spider silks.