P35
Cowpox virus
(CPV) expresses the serpin (serine proteinase inhibitor) CrmA, an
anti-inflammatory, anti-apoptotic protein required for production of red pocks
on chicken chorioallantoic membranes (CAMs). In vitro, CrmA inhibits
several caspases and granzyme
B. Altering the critical P1-aspartate in the CrmA
reactive centre loop to alanine resulted in a virus
(CPV-CrmA-D303A) that resembled CPV deleted for CrmA
(CPVDeltaCrmA : : lacZ); on
CAMs it produced white, inflammatory pocks with
activated caspase-3 and reduced virus yields, suggesting that CrmA activities are mediated via proteinase
inhibition. CrmA in CPV was replaced with SERP2 from Myxoma virus (MYX) or baculovirus
P35, which inhibit similar proteinases in vitro.
SERP2 and P35 each blocked caspase-3-mediated apoptosis but were unable to control inflammation of CAMs.
However, SERP2 and P35 restored virus yields, indicating that the decreased
virus titres seen with CPVDeltaCrmA : : lacZ resulted from apoptosis rather than inflammation. To
compare the activities of CrmA and SERP2 further,
rabbits were infected with MYX recombinant viruses. Intradermal
infection of rabbits with MYX was uniformly lethal, generating raised primary
lesions and many secondary lesions. In contrast, deletion of SERP2 from MYX (MYXDeltaSERP2 : : lacZ) caused
little mortality and produced flat primary lesions with few secondary lesions.
Replacement of SERP2 with CrmA (MYXDeltaSERP2
: : CrmA) resulted in partial complementation
with flat primary lesions, many secondary lesions and death in 70 % of the
rabbits. Therefore, CrmA and SERP2 were not
functionally interchangeable during infection of CAMs
or rabbits, implying that these serpins have
activities that are not evident from biochemical studies with human caspases.
PMID: 15105544 [PubMed - as supplied by publisher]