Structural And Functional Insights Into The Fibroin N-terminal Domain Of Actias Selene

Silk secreted by silk glands in the larval of silkworm are biopolymers which is one of the most remarkable biomaterials with controllable degradation and biocompatibility in nature. It can be classified into two types:mulberry and nonmulberry. Mulberry silk of silkworm Bombyx mori has been extensively explored and used for century old textiles and sutures. However, the transformation of nonmulberry silk from being a textile commodity to biomaterials is relatively new. Fibroins centered by multilayer composite fiber sericin is mainly composed of fibroin heavy chain (FibH), which includes low complexity crystalline domains and non-repetitive N-terminal and C-terminal of fibroin heavy chain. FibH constitutes the skeleton of silk and determines the main physical and chemical properties of silk, so it is very valuable to study the molecular mechanism of aggregation and assembly. The Bombyx mori fibroin molecule, which consists of a H-chain of 390 kDa and a L-chain of 26 kDa connected by a disulfide bond, as well as a glycoprotein named P25 (30 kDa), is secreted into the posterior silk gland as an aqueous solution, which assembled a high molecular mass elementary unit with a 6:6:1 molar ratio. However, Actias selene had "reverted" to a simplified H-H fibroin homodimer without L-fibroin and P25. Our group successfully reported the crytsal structure of fibroin N-terminal domain (FibNT) in Bombyx mori is a homodimer, revealing a remarkable double layered anti-parallel β-sheet with each layer comprising two FibNT molecules entangled together of an eight-stranded P-sheet. However, what’s the fouction and structure of fibroin N-terminal domain of Actias selene (AsFibHN) has been always unkmown. Our team makes every effort to tackle this problem in this study.In this study, by using gel filtration chromatography, SDS-PAGE, DLS, western blotting and NMR (’H-’H NOSEY), AsFibHN was identified be always existing in monomer which is different from BmFibHN dimer that cannot.1H-15N HSQC, CBCANH, CBCA(CO)NH, HNCO, HN(CA)CO, HBHA(CBCACO)NH, H(CCO)NH-TOCSY, (H)CC(CO)NH-TOCSY,15N-NOESY,13C-NOESY, HCCH-COSY and HCCH-TOCSY of AsFibHN (19-105) were successfully recorded. The assignments of the backbone was completed and the determination of three dimensional structure is still undergoing. Based on the chemical shift of HN, HA, CA, CB, CO and N, we predicted the possible secondary structure of AsFibHN (19-105). Hope we could further clarify molecular mechanism of silk secretion in Actias selene, promoting the development of wild silkworm research field.