Ectopic expression of sericin enables efficient production of ancient silk with structural changes in silkworm

Abstract

Bombyx mori silk is a super-long natural protein fiber with a unique structure and excellent performance. Innovative silk structures with high performance are in great demand, thus resulting in an industrial bottleneck. Herein, a transgenic method was used in which the outer layer sericin SER3 in silk is secreted into the inner fibroin layer, thus generating a new structural fiber with non-fibrous sericin microsomes dispersed in fibroin fibrils. The water-soluble SER3 protein secreted by the posterior silk gland causes P25’s detachment from the fibroin unit of the Fib-H/Fib-L/P25 polymer and accumulation on the fibroin surface. Moreover, the water solubility and stability of the fibroin-colloid in the silk glandular cavity are increased, thus significantly improving the β-sheet content of fibroin, as well as the mechanical properties, moisture absorption and moisture liberation of the silk fiber. Our silkworm mutant system circumvents the problems of low vitality and abnormal silk gland development, and enables higher production efficiency of cocoon silk than that of the wild type. We describe a silk gland transgenic target protein selection strategy to alter the ancient silk fiber structure and to innovate the properties of silk protein materials. This study thus provides an efficient, green method to produce new silk fibers.