SPI-1-dependent host range of rabbitpox virus and complex formation with cathepsin G is associated with serpin motifs

Serpins are a superfamily of serine proteinase inhibitors which function to regulate a number of key biological processes including fibrinolysis, inflammation, and cell migration. Poxviruses are the only viruses known to encode functional serpins. While some poxvirus serpins regulate inflammation (myxoma virus SERP1 and cowpox virus [CPV] crmA/SPI-2) or apoptosis (myxoma virus SERP2 and CPV crmA/SPI-2), the function of other poxvirus serpins remains unknown. The rabbitpox virus (RPV) SPI-1 protein is 47% identical to crmA and shares all of the serpin structural motifs. However, no serpin-like activity has been demonstrated for SPI-1 to date. Earlier we showed that RPV with the SPI-1 gene deleted, unlike wild-type virus, fails to grow on A549 or PK15 cells (A. Ali, P. C. Turner, M. A. Brooks, and R. W. Moyer, Virology 202:306-314, 1994). Here we demonstrate that in the absence of a functional SPI-1 protein, infected nonpermissive cells which exhibit the morphological features of apoptosis fail to activate terminal caspases or cleave the death substrates PARP or lamin A. We show that SPI-1 forms a stable complex in vitro with cathepsin G, a member of the chymotrypsin family of serine proteinases, consistent with serpin activity. SPI-1 reactive-site loop (RSL) mutations of the critical P1 and P14 residues abolish this activity. Viruses containing the SPI-1 RSL P1 or P14 mutations also fail to grow on A549 or PK15 cells. These results suggest that the full virus host range depends on the serpin activity of SPI-1 and that in restrictive cells SPI-1 inhibits a proteinase with chymotrypsin-like activity and may function to inhibit a caspase-independent pathway of apoptosis.