Studies on the Nucleocytoplasmic Transport of the E7 Oncoprotein of High-Risk HPV Type 16

Abstract

Human papillomaviruses (HPVs) have been estimated to be the most common sexually transmitted infection in the United States. In addition to condyloma accuminata, infection of the squamous basal epithelium by high-risk HPVs, notably type 16 (HPV16), has been shown to be the primary etiological agent in the majority of cervical carcinomas. The E7 major transforming protein of HPV16, along with E6, has been linked to tumorigenesis and malignancy. While the E7 protein itself possesses no enzymatic activity, its ability to bind a number of nuclear and cytoplasmic targets subverts a variety of cellular regulatory complexes and facilitates viral replication. Previous studies in the Moroianu Lab have shown the HPV16 E7 oncoprotein to translocate across the nuclear pore complex (NPC) in a facilitated manner dependent on a non-canonical, c-terminal, nuclear localization signal (cNLS) for import, and a consensus leucine-rich nuclear export sequence (NES) for export (28). While the leucine-rich NES has been characterized, a full examination of the cNLS has yet to be performed. Here we present evidence that the karyopherin independent nuclear import mediated by the cNLS of 16E7 is dependent on its c-terminal Zn binding domain. Furthermore, we demonstrate that nuclear import is mediated by the direct interaction of a small patch of hydrophobic residues, 65LRLCV69, with the FG domain of the central FG-nucleoporin Nup62. In addition, we examined a potential regulatory mechanism of 16E7 nucleocytoplasmic translocation. Previous work has shown that a serine conserved in the high-risk HPVs at position 71 is phosphorylated by an unknown kinase. Here we present evidence that while phosphorylation of S71 is not required for either 16E7 nuclear localization or nuclear export in HeLa cells, mimicking phosphorylation of the S71 residue results in a statistically significant shift in the distribution of localization phenotypes of the resultant cell population toward a larger percentage exhibiting more nuclear localization. These data suggest that nucleocytoplasmic transport of 16E7 is, at least in part, a regulated process.