The degradation sequence of adenovirus E1A consists of the amino-terminal tetrapeptide Met-Arg-His-Ile

mUBC9, a novel adenovirus E1A-interacting protein that complements a yeast cell cycle defect

Adenovirus E1A encodes two nuclear phosphoproteins that can transform primary rodent fibroblasts in culture. Transformation by E1A is mediated at least in part through binding to several cellular proteins, including the three members of the retinoblastoma family of growth inhibitory proteins. We report here the cloning of a novel murine cDNA whose encoded protein interacts with both adenovirus type 5 and type 12 E1A proteins. The novel E1A-interacting protein shares significant sequence homology with ubiquitin-conjugating enzymes, a family of related proteins that is involved in the proteasome-mediated proteolysis of short-lived proteins. Highest homology was seen with a Saccharomyces cerevisiae protein named UBC9. Importantly, the murine E1A-interacting protein complements a cell cycle defect of a S. cerevisiae mutant which harbors a temperature-sensitive mutation in UBC9. We therefore named this novel E1A-interacting protein mUBC9. We mapped the region of E1A that is required for mUBC9 binding and found that the transformation-relevant conserved region 2 of E1A is required for interaction.

The N-terminal region of the adenovirus type 5 E1A proteins can repress expression of cellular genes via two distinct but overlapping domains

The transforming E1A 12S and E1A 13S proteins of human adenovirus type 5 (Ad5) contain two and three conserved regions, respectively. In the present study, the contribution of sequences in the nonconserved N-terminal region of the E1A proteins to morphological transformation and to down-regulation of a number of mitogen-inducible genes was investigated. As described previously, transformation of NRK cells (an established normal rat kidney cell line) results in denser cell growth and a cuboidal cellular morphology. None of the cells expressing N-terminally mutated E1A proteins, however, show these transformed properties, which suggests an important role for sequences in that domain. The decrease in cyclin D1 levels requires exactly the same sequences. The ability to transform NRK cells and to reduce cyclin D1 levels does not correlate with the presence in the E1A proteins of binding domains for p300, CBP, p107, pRb, cyclin A, or cdk2. In contrast, down-regulation of expression of the JE gene in NRK cells and repression of transcription of the collagenase gene in human HeLa cells does correlate with the presence in the E1A proteins of an intact binding domain for p300 and CBP. The results suggest that the N-terminal domain of the E1A proteins can repress expression of cellular genes by at least two different mechanisms.

Gene expression using xenopus oocytes

Xenopus oocytes have been used as a surrogate genetic system for the study of many facets of gene expression. Some recent salient examples where injected oocytes proved to be indispensable for the analysis of transcription, translation, RNA/protein transport, and ion channel formation are described.

Functions of Intracellular Protein Degradation in Yeast

Diverse vacuolar and nonvacuolar pathways of protein degradation have been described in yeast. In several cases, much is known about the proteases involved, but most of these studies utilized nonphysiological model substrates. On the other hand, many regulatory proteins, such as those involved in cell cycle control, cell type determination, and the regulation of metabolite fluxes through biosynthetic pathways, have been shown to be rapidly and selectively destroyed in vivo, either constitutively or in response to specific regulatory signals. Precisely what molecular features of this class of proteins target them for degradation is largely unknown; this question is an area of intense current interest. A connection has been made between a particular proteolytic mechanism and a specific naturally short-lived protein in only a handful of examples. It is in this regard that the powerful molecular and genetic techniques available in yeast will probably have their greatest impact in the near future. The promise of this type of approach is already becoming apparent with the molecular genetic analysis of the yeast ubiquitin system. Although this work began less than ten years ago, the genes encoding at least 22 proteins involved in ubiquitin-dependent processes have already been isolated, and questions of their physiological and mechanistic function are being answered at an ever quickening pace.