The SH integral membrane protein of the paramyxovirus simian virus 5 is required to block apoptosis in MDBK cells

In some cell types the paramyxovirus simian virus 5 (SV5) causes little cyt opathic effect (CPE) and infection continues productively for long periods of time; e.g,, SV5 can be produced from MDBK cells for up to 40 days with l ittle CPE, SV5 differs from most paramyxoviruses in that it encodes a small (44-amino-acid) hydrophobic integral membrane protein (SH). When MDBK cell s were infected with a recombinant SV5 containing a deletion of the SH gene (rSV5 Delta SH), the MDBK cells exhibited an increase in CPE compared to c ells infected with wild-type SV5 (recovered from cDNA; rSV5). The increased CPE correlated with an increase in apoptosis in rSV5SH-infected cells over mock-infected and rSV5-infected cells when assayed for annexin V binding, DNA content (propidium iodide staining), and DNA fragmentation (terminal de oxynucleotidyltransferase-mediated dUTP-biotin nick end labeling assay). In rSV5 Delta SH-infected MDBK cells an increase in caspase-2 and caspase-3 a ctivities was observed. By using peptide inhibitors of individual caspases it was found that caspase-2 and caspase-3 were activated separately in rSV5 Delta SH-infected cells. Expression of caspase-2 and -3 in rSV5 Delta SH-i nfected MDBK cells appeared not to require STAT1 protein, as STAT1 protein could not be detected in SV5-infected MDBK cells. When mutant mice homologo us for a targeted disruption of STAT1 were used as a model animal system an d infected with the viruses it was found that rSV5 Delta SH caused less mor tality than wild-type rSV5, consistent with the notion of clearance of apop totic cells in a host species.