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Spider silk fibers have remarkable mechanical properties that suggest the component proteins could be useful biopolymers for fabricating biomaterial scaffolds for tissue formation. Two bioengineered protein variants from the consensus sequence of the major component of dragline silk from Nephila clavipes were cloned and expressed to include RGD cell-binding domains. The engineered silks were characterized by CD and FTIR and showed structural transitions from random coil to insoluble beta-sheet upon treatment with methanol. The recombinant proteins were processed into films and fibers and successfully used as biomaterial matrixes to culture human bone marrow stromal cells induced to differentiate into bone-like tissue upon addition of osteogenic stimulants. The recombinant spider silk and the recombinant spider silk with RGD encoded into the protein both supported enhanced the differentiation of human bone marrow derived mesenchymal stem cells (hMSCs) to osteogenic outcomes when compared to tissue culture plastic. The recombinant spider silk protein without the RGD displayed enhanced bone related outcomes, measured by calcium deposition, when compared to the same protein with RGD. Based on comparisons to our prior studies with silkworm silks and RGD modifications, the current results illustrate the potential to bioengineer spider silk proteins into new biomaterial matrixes, while also highlighting the importance of subtle differences in silk sources and modes of presentation of RGD to cells in terms of tissue-specific outcomes.

Recombinant sericin proteins of different molecular masses (17.4, 31.9, and 46.5 kDa), based on the 38-amino acid repetitive motif of native sericin, were cloned, expressed, and purified. The recombinant sericin self-assembled during dialysis (starting concentration of 2.5 mg/ml) forming twisted fibers. Circular dichroism and Fourier transform infrared spectroscopy studies demonstrated protein conformational transitions occurred from random coil to ␤-sheets during the dialysis. Congo red-stained recombinant sericin fibrils exhibited applegreen birefringence, indicating long-range order in the array of ␤-sheets. Biosynthetic sericin has a high content of polar amino acids (e.g. Silk of Bombyx mori is composed of two kinds of proteins: the fibroin which is synthesized in the posterior silk gland and the sericins produced by the middle silk gland. The sericins bind two fibroin fibers together in the anterior region of the gland as the fibers are spun to form the cocoon. Sericins represent a family of proteins whose molecular mass ranges from 20 to 310 kDa (2, 3) and are characterized by an unusually high serine content, 38.1 mol % (4). Sericins are soluble in hot water, which makes it possible to degum the silk fibroin threads.Two genes encode sericins, Ser1 and Ser2 (5-8). The different molecular weight sericins are the products of different splicing events at the transcript level. Some conformation studies with sericins from the silk gland of B. mori or regenerated sericin from B. mori cocoons suggested random coil with some ␤ structure (9, 10). However, the samples used in these studies were mixtures of the various native sericin proteins, therefore the structure of the individual proteins is unknown and the contribution of specific sequences toward secondary structure is not confirmed. Sericin model peptides such as poly(L-serine) (11-16), poly(O-benzyl-L-serine) (12), and poly(O-acetyl-L-serine) (13,14) were synthesized for structural feature characterization. The ability of poly(L-serine) to fold into a ␤ conformation depended on molecular weight. Low molecular weight poly(L-serine) (molecular weight 650) was soluble and adopted a random coil conformation in aqueous solution, whereas high molecular weight poly(L-serine) (molecular weight 105,000) formed ␤-sheets and was insoluble in water (11). However, homopolypeptides like poly(L-serine) do not match native sericin in terms of amino acid composition, sequence, and molecular weight. Therefore, to better understand sericin structure, interactions between sericin and fibroin, and the biological relevance of sericin in fiber structure, high molecular weight pure sericin-like proteins are required.Sericin 1 encoded by Ser 1 consists of 70 repeats of the 38-amino acid motif (5): SSTGSSSNTDSNSNSVGSSTS-GGSSTYGYSSNSRDGSV. The molecular mass of sericin 1 proteins is 76 -284 kDa. The 38-amino acid repeat motif exhibits a high content of hydrophilic amino acids: 44.7% serine, 10.5% threonine, and 7.9% tyrosine, very close to the average amino acid composition of serici...