title: High-risk HPV E6 oncoproteins assemble into large oligomers that allow localization of endogenous species in prototypic HPV-transformed cell lines.
abstract: The E6 oncoproteins of high-risk HPV types 16 and 18 are involved in the development of cervical cancer. Besides its determinant role in carcinogenic progression, HPV E6 oncoprotein has also been instrumental in elucidating fundamental aspects of p53 function and its ubiquitin-proteasome degradation, with counterpart activities in various DNA tumor viruses. Establishing the conformational state and cellular distribution unequivocally for the endogenous protein in HPV-transformed cell lines derived from carcinomas is essential for understanding the underlying mechanism. Recombinant E6 from high-risk strains 16 and 18 folds into soluble oligomers of approximately 1.2 MDa, which are thermostable and display cooperative loss of tertiary and secondary structure upon chemical denaturation. Antibodies raised against these assemblies locate E6 evenly distributed in the cells. By depleting the polyclonal serum by immunoblocking with monomeric E6, the nuclei of Hela and CaSki cells become completely devoid of label, indicating that monomeric species are mainly localized in the nucleus and that both monomers and oligomers share epitopes. The monomeric species promote degradation of p53 by the proteasome, which correlates with the nuclear localization we describe. In contrast, the oligomeric E6 does not promote p53 degradation, in agreement with its cytoplasmic localization inferred from the immunoneutralization experiments. Our results indicate that the cytoplasmic species contain conformational epitopes that may arise from yet undefined homo or hetero-oligomers, but its localization otherwise agrees with that of the other group of major E6 targets, those involving PDZ binding domains, which requires further investigation.
authors.affiliation: Instituto Leloir and CONICET, Patricias Argentinas 435, 1405 Buenos Aires, Argentina.
authors.forename: María M
authors.initials: MM
authors.lastname: García-Alai
authors.forename: Karina I
authors.initials: KI
authors.lastname: Dantur
authors.forename: Clara
authors.initials: C
authors.lastname: Smal
authors.forename: Lía
authors.initials: L
authors.lastname: Pietrasanta
authors.forename: Gonzalo
authors.initials: G
authors.lastname: de Prat-Gay
mesh_terms: D001483:Base Sequence; D002461:Cell Line, Transformed; D002471:Cell Transformation, Neoplastic; D002472:Cell Transformation, Viral; D004279:DNA, Viral / Q000235:genetics; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D005260:Female; D052162:Human papillomavirus 16 / Q000235:genetics / Q000472:pathogenicity / Q000502:physiology; D052161:Human papillomavirus 18 / Q000235:genetics / Q000472:pathogenicity / Q000502:physiology; D006801:Humans; D008954:Models, Biological; D046912:Multiprotein Complexes; D009856:Oncogene Proteins, Viral / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D017510:Protein Folding; D020836:Protein Structure, Quaternary; D012097:Repressor Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D002583:Uterine Cervical Neoplasms / Q000209:etiology / Q000821:virology
chemical_list: D004279:DNA, Viral; D004268:DNA-Binding Proteins; C068651:E6 protein, Human papillomavirus type 16; C052603:E6 protein, Human papillomavirus type 18; D046912:Multiprotein Complexes; D009856:Oncogene Proteins, Viral; D012097:Repressor Proteins; C495901:TP53 protein, human; D016159:Tumor Suppressor Protein p53