Human Papillomavirus Type 18 E6 Protein Binds the Cellular PDZ Protein TIP-2/GIPC, Which Is Involved in Transforming Growth Factor β Signaling and Triggers Its Degradation by the Proteasome

Abstract

In addition to the function they exert in virus replication, viral proteins often strongly interfere with cell physiology by interacting with cellular factors involved in important regulatory networks. Hence, the viral proteins, especially those expressed by DNA or RNA transforming viruses, represent interesting models to better understand cellular regulatory pathways. Along this line, the discovery of the ability of proteins expressed by different viruses to bind various cellular proteins with PSD-95-Discs-large-ZO1 (PDZ) domains has open a new and interesting field of investigation. The PDZ domain is indeed present in many human proteins, as either single or multiple copies, associated or not with other protein domains (for recent reviews see references 4 and 25). The PDZ proteins exert diverse functions, generally having an important organizing role by establishing specific interactions with multiple partners. The archetypal interaction is association of the PDZ domain with the C-terminal end of another protein, with the specificity of the interaction depending on its C-terminal four amino acids and the consensus motif being XS/TXV-COOH for class I PDZ domains (29,32,45). In addition to this general rule, it is now well established that the PDZ domain can also interact with an internal sequence motif and also with another PDZ domain (4, 25). Relying on these properties, PDZ proteins play important roles in transmembrane receptors assembly and clustering, as well as in organization of cellular junctions. This family of proteins also intervenes in signal transduction, and examples of transcriptional regulation activities exist (24, 49). Unexpectedly, it has been observed that three different viral transforming proteins, i.e., E4 open reading frame 1 of adenovirus type 9, E6 of human papillomavirus type 18 (HPV-18), and Tax of human T-cell leukemia virus type 1 (HTLV-1), were able to bind PDZ proteins (31,35,43). These three proteins include a canonical class I PDZ binding site (BS). For E4 and E6, deletion of this C-terminal PDZ BS triggers a loss of in vitro transforming activity (31). The PDZ BS of Tax of HTLV-1 also plays a role in in vitro transformation of Rat-1 cells (23). In line with these observations, it has been observed that a PDZ BS is present at the C terminus of E6 in malignant HPV types, such as HPV-18 or HPV-16, but not in the benign ones, such as . Similarly, Tax of HTLV-2, which is not associated with aggressive leukemia, does not include a functional PDZ BS (23). Collectively, these observations strongly support the notion that a relationship exists between in vivo transformation and interference with the normal functioning of PDZ proteins. Hence, it appears to be important to characterize the complete set of cellular PDZ proteins targeted by these viral oncoproteins. By performing a two-hybrid screen of a human B-lymphocyte cDNA library with Tax as bait, we have previously identified seven different

A PDZ-binding Motif as a Critical Determinant of Rho Guanine Exchange Factor Function and Cell Phenotype

“…Because we were able to detect a significant level of Syx2 expression only in malignant cells, we hypothesize that the less tightly regulated Syx2, which gives rise to a sustained basal level of RhoA activity, up-regulates tumorigenic transcription factors. Association between tumorigenicity and Syx2 expression is suggested also by a previous study, which reported that synectin degradation mediated via its interaction with the viral oncoprotein E6, as well as RNAi silencing of synectin, increased cell proliferation (Favre-Bonvin et al, 2005). Together with the results presented here, this initial data set suggests a novel mechanism of malignant transformation that does not involve truncation of proto-oncogenic Rho GEF (Ron et al, 1989), but rather the expression of an unmodified and loosely targeted Rho GEF splice variant.…”

Selection of cervical keratinocytes containing integrated HPV16 associates with episome loss and an endogenous antiviral response

“…Endogenous activation of antiviral genes in a cell containing a ''selectable'' HPV16 integrant induces a decrease in episome content, with proportionate reduction in episomal expression of E2, E6, and E7. Reduced E6 and E7 will lead to reduced inhibition of type I IFN-inducible genes (19)(20)(21)(22) and the TGF-␤ pathway (23)(24)(25). These events would account for decreased expression of cell cycle genes (17,26), and would also lead to rapid further reduction in episome levels in a positive feedback loop.…”

Transcriptomic analyses of genes differentially expressed by high-risk and low-risk human papilloma virus E6 oncoproteins

“…This is achieved by induction of expression of catalytic subunit of Human Telomerase Reverse Transcriptase (hTERT) by forming a complex with E6-associated protein (E6AP) and binding to hTERT promoter [7,9,10,30,45]. Enhanced expression of hTERT prevents telomere shortening at the end of each cell division, since shortening of telomere length in each cell division corresponds to aging process; therefore constitutive expression of hTERT causes the cell to divide indefinitely causing immortalization.…”