Dragline Silk: A Fiber Assembled with Low-Molecular-Weight Cysteine-Rich Proteins.

Dragline silk has been proposed tocontain two main protein constituents,MaSp1 and MaSp2. However, the mechanical properties of synthetic spidersilks spun from recombinant MaSp1 and MaSp2 proteins have yet to approachnatural fibers, implying the natural spinning dope is missing criticalfactors. Here we report the discovery of novel molecular constituentswithin the spinning dope that are extruded into dragline silk. Proteinstudies of the liquid spinning dope from the major ampullate gland,coupled with the analysis of dragline silk fibers using mass spectrometry,demonstrate the presence of a new family of low-molecular-weight cysteine-richproteins (CRPs) that colocalize with the MA fibroins. Expression ofthe CRP family members is linked to dragline silk production, specificallyMaSp1 and MaSp2 mRNA synthesis. Biochemical data support that CRPmolecules are secreted into the spinning dope and assembled into macromolecularcomplexes via disulfide bond linkages. Sequence analysis supportsthat CRP molecules share similarities to members that belong to thecystine slipknot superfamily, suggesting that these factors may haveevolved to increase fiber toughness by serving as molecular hubs thatdissipate large amounts of energy under stress. Collectively, ourfindings provide molecular details about the components of draglinesilk, providing new insight that will advance materials developmentof synthetic spider silk for industrial applications. [ABSTRACT FROM AUTHOR]