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Journal of Virology, November 1999, p. 8999-9010, Vol. 73, No. 11
Department of Molecular Genetics and
Microbiology, University of Florida, Gainesville, Florida
32610-0266
Received 2 April 1999/Accepted 16 July 1999
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.
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
SPI-1-Dependent Host Range of Rabbitpox Virus and Complex
Formation with Cathepsin G Is Associated with Serpin Motifs
*
Corresponding author. Mailing address: Department of
Molecular Genetics and Microbiology, College of Medicine, University of
Florida, Box 100266, Gainesville, FL 32610-0266. Phone: (352) 392-7077. Fax: (352) 846-2042. E-mail:
rmoyer{at}medmicro.med.ufl.edu.
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