Expression, purification, and functional characterization of adenovirus 5 and 12 E1A proteins produced in insect cells

Comprehensive sequence analysis of the E1A proteins of human and simian adenoviruses

Despite extensive study of human adenovirus type 5 E1A, surprisingly little is known about the E1A proteins of other adenoviruses. We report here a comprehensive analysis of the sequences of 34 E1A proteins. These represent all six primate adenovirus subgroups and include all human representatives of subgroups A, C, E, and F, eight from subgroup B, nine from subgroup D, and seven simian adenovirus E1A sequences. We observed that many, but not all, functional domains identified in human adenovirus type 5 E1A are recognizably present in the other E1A proteins. Importantly, we identified highly conserved sequences without known activities or binding partners, suggesting that previously unrecognized determinants of E1A function remain to be uncovered. Overall, our analysis forms a solid foundation for future study of the activities and features of the E1A proteins of different serotypes and identifies new avenues for investigating E1A function.

Regeneration of the binding properties of adenovirus 12 early region 1A proteins after preparation under denaturing conditions

Adenovirus 12 early region 1A (Ad12 E1A) was expressed in Escherichia coli. Protein was purified in good yield in the presence of 8 M urea and then renatured by dialysis against dilute NH4HCO3 buffer. The affinity of this protein for pRb, C-terminal binding protein (CtBP), TATA binding protein (TBP), and SUG1 was similar to, or greater than, that of Ad12 E1A prepared by immunoaffinity chromatography under nondenaturing conditions. While the binding of the 266- and 235-amino-acid (aa) E1A components to TBP showed similar characteristics the larger E1A protein had a higher affinity for CtBP, pRb, and SUG1. Using nuclear magnetic resonance (NMR) spectroscopy it was shown that structural perturbations occurred in the 266-aa protein in the presence of Zn2+ consistent with binding--no such changes were seen for the 235-aa protein. Limited proteolysis of the 266- and 235-aa E1A proteins gave rise to comparable polypeptide products, suggesting overall similarities in structure. However, the different affinities of the 266- and 235-aa proteins for the partner proteins and the differences seen in the NMR spectra from the two proteins suggested structural differences.

Adenovirus E1a interferes with expression of vaccinia viral genes

The 12S and 13S cDNAs of the oncogene E1a encoded by the early region of adenovirus 12 (Ad12) were overexpressed using the T7/encephalomyocarditis (EMC)/vaccinia hybrid expression system. The E1a proteins were stable for at least 12 h in monkey epithelial BSC1 cells. The E1a proteins were recognized by a rabbit polyclonal antibody and displayed phosphorylation patterns similar to those displayed by the E1a proteins expressed in Ad12-transformed cells. Expression of E1a proteins by recombinant vaccinia virus led to inhibition of vaccinia viral protein synthesis which was observed as soon as 6 h after infection. This suppression was mediated by both the 12S and the 13S products of Ad12E1a and to a somewhat lesser extent by the 13S product of Ad2E1a. The inhibition of vaccinia virus gene expression resulted in enhanced survival of vaccinia virus-infected cells. These results suggest that the proteins encoded by the E1a sequester a viral or a cellular product(s) that is essential for the expression of vaccinia virus-encoded genes.

The xeroderma pigmentosum group B protein ERCC3 produced in the baculovirus system exhibits DNA helicase activity

The XPB/ERCC3 gene corrects the nucleotide excision-repair defect in the human hereditary disease xeroderma pigmentosum group B and encodes the largest subunit of the basal transcription factor BTF2/TFIIH. The primary sequence of the XPB/ERCC3 protein features the hallmarks of seven helicase motifs found in many known and putative helicases or helicase-related proteins. Recently, the multiprotein BTF2/TFIIH complex has been found to be associated with DNA helicase activity. To explore the properties and functions of XPB/ERCC3, we have used the baculovirus/insect-cell expression system to produce recombinant protein. We report here the construction and analysis of recombinant baculovirus expressing XPB/ERCC3. The XPB/ERCC3 protein is synthesized at a relatively high level in baculovirus-infected insect cells. While the majority of XPB/ERCC3 end up in the insoluble fraction of insect cell lysates, a minor fraction of recombinant protein is present in soluble form which can be purified under native conditions. We have found that a DNA helicase activity is associated with the purified XPB/ERCC3 protein, suggesting that XPB/ERCC3 may function as a DNA helicase in local unwinding of DNA template both in the context of transcription and nucleotide excision repair.

Transcription stimulation of the adenovirus type 12 E1a gene in vitro by the 266-amino-acid E1A protein

We previously showed that the 13S but not the 12S mRNA product of the E1a gene of the highly oncogenic type 12 adenovirus (Ad12) stimulates the expression of its own gene. In this study, the mechanism for the autoregulation of the Ad12 E1a gene was investigated in vitro. The 266-amino-acid E1A protein of Ad12 was synthesized in yeast cells and purified as a 57-kDa polypeptide. The purified Ad12 E1A protein stimulated transcription from the proximal promoter of its own gene but had almost no effect on that from the distal promoter. A 35-bp upstream region including a TATA box for the proximal promoter seemed to be sufficient for transcription stimulation by the E1A protein. The Ad12 E1A protein formed a complex with a TATA box-binding protein (TBP), as does the E1A protein of nononcogenic Ad serotypes. Moreover, the E1A protein significantly reduced the binding of TBP to a TATA sequence, while it did not affect the DNA-binding activity of nuclear factor I, a stimulatory protein of the distal transcription of the Ad12 E1a gene. These results suggest that the 13S mRNA product of the Ad12 E1a gene regulates the transcription of its own gene by modulating the activity of TBP.

A- and B-type cyclins differentially modulate substrate specificity of cyclin-cdk complexes

Both cyclins A and B associate with and thereby activate cyclin-dependent protein kinases (cdks). We have investigated which component in the cyclin-cdk complex determines its substrate specificity. The A- and B-type cyclin-cdk complexes phosphorylated histone H1 and their cyclin subunits in an indistinguishable manner, irrespective of the catalytic subunit, p33cdk2 or p34cdc2. In contrast, only the cyclin A-cdk complexes phosphorylated the Rb-related p107 protein in vitro. Likewise, binding studies revealed that cyclin A-cdk complexes bound stably to p107 in vitro, whereas cyclin B-cdk complexes did not detectably associate with p107, under identical assay conditions. Binding to p107 required both cyclin A and a cdk as neither subunit alone bound to p107. These results demonstrate that although the kinase subunit provides a necessary component for binding, it is the cyclin subunit that plays the critical role in targeting the complex to p107. Finally, we show that the cyclin A-p33cdk2 complex phosphorylated p107 in vitro at most of its sites that are also phosphorylated in human cells, suggesting that the cyclin A-p33cdk2 complex is a major kinase for p107 in vivo.

Adenovirus-E1A proteins transform cells by sequestering regulatory proteins

Cell transformation by adenovirus-E1A proteins is mediated by binding to cellular proteins whose functions are thereby inactivated or altered. The various properties of the E1A proteins are reviewed in relation to their binding to cellular proteins. A number of the cellular proteins which associate to E1A have been identified: the retinoblastoma-susceptibility protein (Rb), the p107 protein, cyclin A and the p33cdk2 kinase. Recent data have shown that those proteins are also able to bind to transcription factor E2F. Binding of Rb to E2F represses the transcription-activating potential of E2F. E1A can sequester the regulatory proteins, like Rb, and thereby release free, active E2F. The domains in E1A that are essential for this transcriptional regulation are also required for the transforming properties of E1A.