The viral serpin SPI-1 inhibits the host cell serine protease FAM111A

Authors

• AL Welter
• S Palani
• Y Machida
• MJ Schellenberg
• SH Kaufmann
• YJ Machida

Abstract

Host-range mutant viruses that inefficiently replicate in restrictive cells are useful tools for understanding the interplay between host-virus interactions, as it becomes easier to study the host factors important for defending cells against viral infections. Deletion of the gene encoding the viral protein, serine protease inhibitor 1 (SPI-1), a member of the serpin family of serine protease inhibitors, generates a host-range defect in orthopoxvirus infection of restrictive cells. Serpins inhibit their target enzymes irreversibly through covalent complex formation with their reactive center loop (RCL) and the protease, followed by a large conformational change within both proteins. Although SPI-1 is a serpin protein, it has yet to be demonstrated as inhibitory and it is unknown whether it inhibits host cell proteases to promote viral infection. A previous study reported that knock down of the host cell serine protease FAM111A allows replication of a host-range mutant virus lacking the SPI-1 gene. Therefore, it was hypothesized that SPI-1 inhibits FAM111A’s protease activity, however this is yet to be demonstrated. Here, we show that SPI-1 directly inhibits FAM111A’s protease activity in vitro. Our biochemical study suggests that SPI-1 forms a covalent complex with FAM111A in an active-site-dependent manner, demonstrating that SPI-1 inhibits FAM111A through the irreversible covalent bond formation characteristic to serpin inhibition. Additionally, we found residues within SPI-1’s RCL are critical for rabbitpox virus infection in vivo. Altogether, our findings identify SPI-1 as the first protease inhibitor of FAM111A and suggest that SPI-1 blocks FAM111A’s ability to function as a host restriction factor.

Scientific Focus Area: Molecular Biology and Biochemistry

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