Cystal structure of inhibitor-bound human MSPL/TMPRSS13 that can activate high pathogenic avian influenza

A viral surface envelope glycoprotein, hemagglutinin (HA), is cleaved by host cell proteins of transmembrane protease serine (TMPRSS) family, that triggers a viral infection. The extracellular region of TMPRSS-2, -3, -4, and MSPL are composed of LDLA, SRCR, and SPD domains. MSPL can cleave the consensus multibasic (R-X-X/R-R) and monobasic (Q(E)-T/X-R) motifs on the HA, while TMPRSS2 or -4 cleaves monobasic motifs only. To elucidate the HA cleavage of the recognition motif by MSPL, we solved the crystal structure of extracellular region of human MSPL in complex with the furin inhibitor. The structure revealed that three domains are gathered around the C-terminal alpha-helix of SPD domain. Furin inhibitor structure shows that the side chain of P1-Arg inserts into highly conserved S1 pocket, whereas side chain of P2-Lys interacts with the Asp/Glu-rich 999s loop that is unique to MSPL. Based on our structure, we designed four inhibitors which showed more specifically to MSPL than the furin inhibitor. We also constructed a homology model of TMPRSS2, that is identified as an initiator of SARS-CoV-2 infection, suggested that TMPRSS2 is more suitable for Ala/Val residues at P2 site than Lys/Arg residues.