Serine protease Bm-SP142 was differentially expressed in resistant and susceptible Bombyx mori strains, involving in the defence response to viral infection

Bm-SP142 is a 35 kDa protease in the silkworm, but its exact functions remain unknown. In this study, sequence alignment revealed that the His-Asp-Ser catalytic triad is embedded in the TAAHC-DIAL-GDSGGP sequence motif, establishing Bm-SP142 as a serine protease. Soluble recombinant GST-BmSP142 was expressed and purified, and serine protease activity was confirmed in vitro. RT-qPCR results indicated that Bm-SP142 was mainly expressed in the middle part of the silkworm midgut, and Bm-SP142 transcripts were significantly up-regulated at 24 hours post infection (hpi) in BmBDV-resistant strains (798) inoculated with BmBDV and BmNPV-resistant strains (NB) inoculated with BmNPV, but not in BmBDV-susceptible strains (306). Surprisingly, transcripts were significantly down-regulated at 12 hpi in BmNPV-susceptible strains (HuaBa 35) inoculated with BmNPV, compared with healthy silkworms. Recombinant BmNPV treated with purified Bm-SP142 effectively impaired its ability to infect BmN cells, and Bm-SP142 decreases the efficiency of BmNPV and BmBDV propagation in silkworms. Furthermore, overexpression of Bm-SP142 in BmN cells inhibited viral propagation.