Disease relevance of SPI3

• The orthopoxvirus serpin SPI-3 is N-glycosylated and suppresses fusion between infected cells [1].
• The cowpox virus serpin SPI-3 complexes with and inhibits urokinase-type and tissue-type plasminogen activators and plasmin [1].
• As a proteinase inhibitor, SPI-3 has similar specificity to the leporipoxvirus SERP1 protein of myxoma virus, although the two serpins are less than 30% identical overall [1].
• SPI-3 inhibits cell-cell fusion during infections with both CPV and vaccinia virus [2].
• The cowpox virus SPI-3 and myxoma virus SERP1 serpins are not functionally interchangeable despite their similar proteinase inhibition profiles in vitro [3].

High impact information on SPI3

• The orthopoxviruses cowpox virus and rabbitpox virus each encode three members of the serpin family of proteinase inhibitors, designated SPI-1, SPI-2, and SPI-3 [4].
• Therefore, SPI-3 does not appear to control cell fusion by acting as a serine proteinase inhibitor [2].
• Treatment with the N glycosylation inhibitor tunicamycin converted the two SPI-3 proteins to a single 40-kDa protein, close to the size of 42 kDa predicted from the DNA sequence, suggesting that the SPI-3 protein, unlike the other two orthopoxvirus serpins, is a glycoprotein [2].
• Site-directed mutations were constructed near the C terminus of SPI-3, in or near the predicted reactive-site loop which is conserved in inhibitory serpins [2].
• A transfection assay was devised to test engineered mutants of SPI-3 for the ability to inhibit fusion [2].

Chemical compound and disease context of SPI3

• Fusion triggered by SPI-3 mutants, like that for HA- mutants, was inhibited by the monoclonal antibody C3 directed against the vaccinia virus 14-kDa envelope protein [5].

Biological context of SPI3

• Although SPI-3 contains motifs conserved in inhibitory serpins, no proteinase inhibition by SPI-3 has been demonstrated, and mutations within the serpin reactive center loop (RCL) do not affect the ability to regulate cell fusion [1].
• Immunoblotting with an anti-SPI-3 antibody showed that the SPI-3 protein is synthesized early in infection prior to DNA replication [2].
• SPI-3-negative strains fused despite the fact that the HA was expressed and processed normally, i.e., cells infected with SPI-3 mutants remained functionally hemadsorption positive, and analysis of the HA protein by Western immunoblot suggested that posttranslational modifications of the HA protein appeared normal [5].
• Although SPI-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 inhibitory 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, 267-280] [3].

Anatomical context of SPI3

• SPI-3 and HA from CPV infected cells co-localize to the plasma membrane and are found in extracellular enveloped virus (EEV) [6].

Associations of SPI3 with chemical compounds

• We demonstrate here that SPI-3 protein expressed by transcription/translation in vitro is able to form SDS-stable complexes with the serine proteinases plasmin, urokinase-type plasminogen activator (uPA), and tissue-type plasminogen activator (tPA), consistent with inhibitory activity of the serpin [1].
• Mutation of Arg-340/Ser-341 at the predicted P1/P1' sites within the RCL prevented the formation of complexes between SPI-3 and plasmin, uPA, or tPA, suggesting that the arginine at the P1 position was required for complex formation [1].

References