Identification of Ser2 proteins as major sericin components in the non-cocoon silk of Bombyx mori

Sericins are glue proteins of Bombyx mori silk fibers. They are produced in the middle silk gland (MSG) cells, stored in the lumen, and pushed out from the spinneret surrounding the fibroin fibers. The Ser2 gene was partly cloned from the anterior region of the MSG more than 20 years ago and is regarded as a sericin-encoding gene; however, Ser2 proteins do not appear to be major components of cocoon proteins. We used northern blotting to analyze the expression of three sericin genes—Ser1, Ser2, and Ser3—in the MSG of third to fifth instar larvae and measured the corresponding cDNA levels by competitive PCR. The results revealed that Ser2 gene expression dominated until the middle period of the fifth instar, while the expression of the other genes was mainly observed after the middle fifth instar. Protein analysis demonstrated that the two Ser2 proteins produced by alternative splicing were major coating proteins of larval silk threads spun during the growing stages. The molecular components of larval silk sericin were completely different from those of cocoon sericin, and the difference may be related to the functions of the two kinds of silk fibers.