The human cytomegalovirus (HCMV) UL37 immediate-early (IE) gene minimally encodes three protein isoforms that share NH2-terminal sequences. The predominant UL37 isoform detected during HCMV infection was the UL37 exon 1 protein (pUL37x1), which was produced from IE and, more abundantly, through late times of infection. pUL37x1 was localized in both the endoplasmic reticulum (ER) and mitochondria in infected cells. To determine which UL37x1 NH2-terminal residues serve as ER and mitochondrial targeting signals, we examined the subcellular localization of two deletion mutants. pUL37x1Δ2–23, which lacks the hydrophobic leader, is neither translocated into the ER nor imported mitochondrially; conversely, pUL37x1Δ23–34, lacking the juxtaposed basic residues, was translocated into the ER but only imported weakly into mitochondria. These studies show for the first time the temporal production and localization of pUL37x1 during HCMV infection. The trafficking patterns of mutants suggest that the pUL37x1 targeting signal to ER and mitochondria is bipartite.
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