The purpose of this study was to investigate the significance of the C- terminal RDEL motif of the myxoma virus M-T4 protein in terms of apoptosis regulation and role in viral virulence. To accomplish this, a recombinant myxoma virus was created in which the C-terminal RDEL motif of M-T4 was deleted and a selectable marker (Ecogpt) was inserted immediately downstream. We hypothesized that removal of the RDEL motif from M-T4 would alter the subcellular localization of the protein and provide insight into its antiapoptotic role. Surprisingly, removal of the RDEL motif from M-T4 did not affect localization of the protein within the endoplasmic reticulum (ER), but it did reduce the stability of the mutant protein. Pulse-chase immunoprecipitation and endoglycosidase H analysis coupled with confocal fluorescent light microscopy demonstrated that the M-T4 RDEL- mutant protein is retained in the ER like wildtype M-T4 and suggests that the C-terminal RDEL motif is not the sole determinant for M-T4 localization to the ER. Infection of cultured rabbit lymphocytes with the M-T4 RDEL- mutant virus results in an intermediate apoptosis phenotype compared with the wildtype and M-T4 knockout mutant viruses. A novel myxomatosis phenotype was observed in European rabbits when infected with the recombinant M-T4 RDELmutant virus. Rabbits infected with the M-T4 RDEL- virus on day 9 postinfection exhibited an exacerbated edematous and inflammatory response at secondary sites of infections, particularly the ears. Our results indicate that the C-terminal RDEL motif may not be solely responsible for retention of M-T4 to the ER and that M-T4 may have a dual function in protecting infected lymphocytes from apoptosis and in modulating the inflammatory response to virus infection.
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