Evidence of a Calcium-Chelating Protein Involved in Silkworm Silk Fiber Spinning.

Abstract

Silk cocoons consist primarily of fibroin and sericin but also contain hundreds of uncharacterized proteins with unknown biological function. This study characterizes a silk extract produced using a recently developed extraction method that preserves native protein structure and composition, revealing proteins that are likely lost or overlooked using conventional extraction approaches. As expected, it extracted the primary fibroin and sericin proteins, but unexpectedly, the extract was dominated by sericin 3 and an 18.4 kDa protein with unknown function. We propose two possible functions: firstly, that it facilitates sericin 3 expression or secretion and secondly, that it acts as a "calcium chelator", acting as a molecular trigger for fiber formation. In addition, analysis of the theoretical fractional charges of the primary silk proteins predicts that sericin 3 and fibrohexamerin (P25) transitions from negatively charged to neutral charge at the pH in the anterior silk gland, suggesting a previously unrecognized mechanism for regulating protein interactions during spinning. Together, these findings point towards an explanation for how silkworms can store silk proteins at high concentrations in the gland without aggregation then selectively destabilize the mixture on demand during fiber spinning. This understanding could have significant implications for the development of regenerated silk feedstocks for artificial silk spinning.