The function of one type of Kazal-type serine protease inhibitor from Silkworms, Bombyx mori (Lepidoptera)

Abstract

The silk of silkworm, Bombyx mori, consists mainly of proteins, which contain some small proteins with specialized functions that confer special properties to silk, such as antimicrobial properties. Serine protease inhibitors (SPIs) are one of them, and the types that have been identified are seroin, serpin, Kunitz type, and Kazal type, but the function of the Kunitz/Kazal type is not known. As compared to others, Kazal molecular weight is relatively minimal. In this study, we focused on the function of a Kazal-type inhibitor BmSPI2, and its sequence characterization and expression profile were first refined. To clarify its function, BmSPI2 recombinant protein was expressed in sf9 cells using baculovirus expression system. Further testing revealed that BmSPI2 could inhibit trypsin, α-chymotrypsin, and plasmin, whereas has only some effect on proteinase K. This suggests that BmSPI2 has significant serine protease inhibitory activity. In-depth analysis revealed that BmSPI2 also effectively inhibited fibrinogen degradation mediated by fibrinolytic enzymes. Finally, we examined its antimicrobial-related functions using prepared antibodies and found that BmSPI2 was also able to directly bind to bacteria and fungi to inhibit their proliferation. The results of the study enriched the function of BmSPI2 and laid a theoretical foundation for its development and utilization.