During storage in the silk gland, the n-terminal domain (nt) of spider silk proteins (spidroins) keeps the aggregation-prone repetitive region in solution at extreme concentrations. We observe that nts from different spidroins have co-evolved with their respective repeat region, and now use an NT that is distantly related to previously used NTs, for efficient recombinant production of the amyloid-β peptide (Aβ) implicated in Alzheimer's disease. A designed variant of nt from Nephila clavipes flagelliform spidroin, which in nature allows production and storage of β-hairpin repeat segments, gives exceptionally high yields of different human Aβ variants as a solubility tag. This tool enables efficient production of target peptides also in minimal medium and gives up to 10 times more isotope-labeled monomeric Aβ peptides per liter bacterial culture than previously reported.Orb-weaving spiders manufacture up to seven different silks, e.g. dragline silk derived from major ampullate silk proteins (spidroins, MaSp) and flagelliform silk derived from flagelliform spidroins (FlSp). The various spidroins share a common architecture -a large core repetitive region capped by globular N-and C-terminal domains (NT and CT) 1 . The divergent and large aggregation-prone repetitive regions of the spidroins determine the mechanical properties of the respective spider silks, while the terminal domains regulate silk fiber formation 2,3 . Despite their high aggregation propensity the spidroins can be stored at extremely high concentrations (30-50% w/v) in the spider silk gland, solubilized by the NT domain 1,4 .The NT dimerizes upon a drop in pH, which is crucial for silk fiber formation 1,5 . To ensure solubility also at low pH and widen the applicability of NT as a solubility enhancing fusion partner, a charged-reversed mutant has been designed (referred to as NT* MaSp ) 6 . The previously reported NT* MaSp tag is derived from the NT domain of Euprosthenops australis MaSp1 and folds as a five-helix bundle 6,7 . NT* MaSp is a pH insensitive constitutive monomer, highly stable and extremely soluble, and has been successfully applied for efficient production and purification of, among others, lung surfactant protein analogs, cholecystokinin-58, human antimicrobial cathelicidin and a designed β-sheet protein 6,8 .Aggregation-prone proteins and peptides are associated with several neurodegenerative disorders, e.g. Alzheimer's disease (AD), the most prevalent form of dementia 9,10 . These proteins/peptides often exhibit high β-sheet propensity, which make them prone to aggregate and form insoluble amyloid fibrils 11 . These intrinsic properties of amyloid-forming proteins make high-yield biochemical production challenging, yet the availability of pure protein samples is crucial for studying protein self-assembly and its associated neurotoxicity in vitro and in vivo. This is probably one important reason behind the fact that, despite immense efforts, the exact mechanisms of Aβ self-assembly are still unknown 9-11 . Recent advances have howe...

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