The cowpox virus SPI-3 and myxoma virus SERP1 serpins are not functionallyinterchangeable despite their similar proteinase inhibition profiles in vitro

The myxoma virus (MYX) serpin SERP1 is a secreted glycoprotein with anti-in flammatory activity that is required for full MYX virulence in vivo. The co wpox virus (CPV) serpin SPI-3 (vaccinia virus ORF K2L) is a nonsecreted gly coprotein that blocks cell-cell fusion, independent of serpin activity, and is not required for virulence of vaccinia Virus or CPV in mice. Although S PI-3 has only 29% overall identity to SERP1, both serpins have arginine at the P1 position in the reactive center loop, and SPI-3 has a proteinase inh ibitory profile strikingly similar to that of SERP1 [Turner. P. C., Baquero , M. T., Yuan, S., Thoennes, S. R., and Moyer, R. W. (2000) Virology 272, 2 67-280]. To determine whether SPI-3 and SERP1 were functionally equivalent, a CPV variant was constructed where the SPI-3 gene was deleted and replace d with the SERP1 gene regulated by the SPI-3 promoter. Cells infected with CPV Delta SPI-3::SERP1 secrete SERP1 and show extensive fusion, suggesting that SERP1 is unable to functionally substitute for SPI-3 in fusion inhibit ion. In the reciprocal experiment, both copies of SERP1 were deleted from M YX and replaced with SPI-3 under the control of the SERP1 promoter. Cells i nfected with the MYX Delta SERP1::SPI-3 recombinant unexpectedly secreted S PI-3, suggesting either that the cellular secretory pathway is enhanced by MYX or that CPV encodes a protein that prevents SPI-3 secretion. MYX Delta SERP1::SPI-3 was as attenuated in rabbits as MYX Delta SERP1::lacZ, indicat ing that SPI-3 cannot substitute for SERP1 in MYX pathogenesis. (C) 2000 Ac ademic Press.