Functional analysis of the human papillomavirus type 16 E1=E4 protein provides a mechanism for in vivo and in vitro keratin filament reorganization.

High-risk human papillomaviruses, such as human papillomavirus type 16 (HPV16), are the primary cause of cervical cancer. The HPV16 E1=E4 protein associates with keratin intermediate filaments and causes network collapse when expressed in epithelial cells in vitro. Here, we show that keratin association and network reorganization also occur in vivo in low-grade cervical neoplasia caused by HPV16. The 16E1=E4 protein binds to keratins directly and interacts strongly with keratin 18, a member of the type I intermediate-filament family. By contrast, 16E1=E4 bound only weakly to keratin 8, a type II intermediate-filament protein, and showed no detectable affinity for the type III protein, vimentin. The N-terminal 16 amino acids of the 16E1=E4 protein, which contains the YPLLXLL motif that is conserved among supergroup A viruses, were sufficient to target green fluorescent protein to the keratin network. When expressed in the SiHa cervical epithelial cell line, the full-length 16E1=E4 protein caused an almost total inhibition of keratin dynamics, despite the phosphorylation of keratin 18 at serine 33, which normally leads to 14-3-3-mediated keratin solubilization. Mutant 16E1=E4 proteins which lack the LLKLL motif, or which have lost amino acids from their C termini, and which were compromised in the ability to associate with keratins did not disturb normal keratin dynamics. 16E1=E4 was found to exist as dimers and hexamers, whereas a C-terminal deletion mutant (16E1=E4Delta87-92) existed as monomers and formed multimeric structures only poorly. Considered together, our results suggest that by associating with keratins through its N terminus, and by associating with itself through its C terminus, 16E1=E4 may act as a keratin cross-linker and prevent the movement of keratins between the soluble and insoluble compartments. The increase in avidity associated with multimeric binding may contribute to the ability of 16E1=E4 to sequester its cellular targets in the cytoplasm.