∧ 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 ∧ E4⌬87-92) 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.Human papillomaviruses (HPVs) are small double-stranded DNA viruses of ϳ8,000 bp. They infect stratified epithelium and produce lesions that range in severity from benign warts to invasive carcinomas (24). HPV DNA has been detected in Ͼ99.7% of cervical cancers, with HPV16 occurring most frequently (28,29,44). HPV16 is a high-risk HPV type which causes cervical lesions that can progress to high-grade neoplasia and cancer (43).The life cycle of HPVs is closely linked to the differentiation status of the host epithelium. After infecting basal cells through a wound, the viral genome maintains itself episomally at a low copy number (5). As the infected cell migrates toward the epithelial surface and undergoes terminal differentiation, the productive stages of the viral life cycle are triggered. Vegetative viral DNA replication is followed by the expression of capsid proteins and the assembly of infectious virions in the superficial cell layers (24).The HPV16 E1 ∧ E4 protein is expressed in abundance during the late stages of the virus life cycle in the upper layer of the epithelium and coincides with the onset of viral genome amplification (12,26,35). Although the precise role of 16E1 ∧ E4 is unclear, previous work has revealed that 16E1 ∧ E4 can induce cell cycle arrest in G 2 (7), can bind to a DEAD box RNA helicase (E4-DBP) (9), and, when expressed in cultured epithelial cells, can interact with keratins and cause the reorganization of the keratin intermediate-filament network (11). Although the mechanism by which 16E1 ∧ E4 mediates keratin filament reorganization is not understood, immunofluorescence staining has shown the LLKLL motif located close to the N terminus to be necessary for filament colocalization and has shown the C terminus to be necessary for filament collapse.Keratins are major structural proteins in epithelial cells and form the cytoplasmic network of intermediate filaments (17). They contain at least 20 members, called keratin 1 (K1) to K20, which are divided into two types according to the sequence...

No abstract

No abstract

No abstract

No abstract