Virus Research (May 2022)
Identification of Marek's disease virus pUL56 homologue and analysis of critical amino acid stretches indispensable for its intracellular localization
Abstract
Marek's disease virus (MDV) is considered a unique member of the Alphaherpesvirinae subfamily that induces rapid onset of T cell lymphoma in chickens. Compared with other conserved UL56 gene homologues of herpesviruses, little is known about the roles of MDV UL56 gene, while recent studies of mammalian herpesvirus pUL56 proteins have revealed their involvement in promoting ubiquitination of the Nedd4 (neural precursor cell expressed developmentally down-regulated protein 4) -like E3 ubiquitin ligases for proteasomal degradation and in modulating host immune responses. To determine the expression kinetics of UL56 gene products, chicken embryo fibroblasts were infected with very virulent or attenuated MDV strain and analyzed by quantitative PCR and Western blotting. During the time course of infection, the levels of UL56 mRNA transcripts increased consistently. At the translational level, the pUL56 protein encoded by UL56 gene was expressed in the size of 32 kDa, which emerged as early as 12 h post-infection (hpi) but otherwise began to wane at 72 hpi thereafter. With the treatment of viral DNA synthesis inhibitors, the pUL56 expression was significantly reduced, featuring the dynamics of a late (γ)-gene product. By confocal imaging, pUL56 was found to reside in the Golgi compartment. Both the L-domain motifs and the C-terminal tail-anchored transmembrane were essential for its intracellular localization. Noticeably, pUL56 co-localized with a truncated mutant of the chicken Nedd4-like family protein harboring only the WW domains; however, co-immunoprecipitation assay established no direct interaction between them, and the ectopic expression of pUL56 did not alter the abundance of endogenous Nedd4-like protein. Overall, the present study provides a caveat that the pUL56 homologues of different herpesviruses with structural similarities might vary in expression patterns and probably in functional consequences. For this reason, further investigation should be encouraged to focus on the potential association between UL56 gene and MDV pathogenesis in the context of engineered viral mutants.
