Structural model of an antistasin/notch-like fusion protein from the cocoon wall of the aquatic leech, Theromyzon tessulatum.

The aquatic leech, Theromyzon tessulatum, secretes a proteinaceous cocoon with extraordinary physical properties (e.g., proteolytic, thermal resiliency). The deduced amino acid sequence of a major protein (Tcp-Theromyzon cocoon protein) from the T. tessulatum cocoon wall has been used to model the endogenous structure of the Tcp protein. The Tcp protein sequence comprises six internal repeats, each containing 12 ordered Cys residues. Amino acid alignments suggest that the region Cys1-->6 is homologous to antistasin, a leech anticoagulant, and Cys7-->12 is homologous to an epidermal growth factor-like domain found in notch-class proteins, which play critical roles in development, signaling, and adhesion throughout the Animalia. Modeling of individual domains (i.e., antistasin and notch) positions multiple hydrophobic and charged residues on the surface. When the antistasin and notch domains were fused, hydrophobic pockets appeared that may facilitate a polymerization mechanism. Collectively, the predicted features of our Tcp model are consistent with the physical properties of the leech cocoon wall.