Final Activity Report Summary - SSEXSY (Spider silk extrusion systems) The c-terminal sequences of spidroins derived from different glands are non-repetitive and highly conserved. These close relationships underline the high importance of these peculiar sequences and suggest a special role the c-termini play for the production and properties of the silks. Several functions have been attributed to the c-terminal sequences of major ampullate silks. They might represent signal peptides or be necessary for the solubility of the spidroins in the highly concentrated spinning dope. We analysed the c-termini of spidroins derived from the major ampullate gland of spiders of the genus nephila. Their secondary structures and biochemical data of expressed fusion proteins predicted that the presence of c-termini influenced the conformation and solubility of spidroins. Our sequence and biochemical analyses argued for a structural role the c-termini played for the spidroins. The fact that recombinant c-terminal sequences formed dimers under non-reducing conditions and the lack of dimerisation of the similar structured MiSps lacking the c-terminal cysteine strongly suggested that the latter contributed to divalent intermolecular crosslinks in the Masps. Since a higher molecular weight material in non-reducing conditions was seen within the gland sac, the storage compartment of the gland and solubilised threads alike, disulfide bridge formation seemed to be already triggered shortly after spidroin synthesis and maintained during fiber spinning, which emphasised the importance of this posttranslational modification. Crosslinks introduced by disulfide bridge formation would certainly influence the alignment of spidroins in the process of crystallisation during the spinning process.
