Expression of biologically active human interferon alpha 2 in Aloe vera.

Methods necessary for the successful transformation and regeneration of Aloe vera were developed and used to express the human protein, interferon alpha 2 (IFNα2). IFNα2 is a secreted cytokine that plays a vital role in regulating the cellular response to viral infection. Transgenic plants were regenerated from callus cultures initiated from zygotic embryos. Expression of the IFNA2 transgene in transformed plants was confirmed by RT-PCR and IFNα2 protein was detected by immunoblot analysis. Human A549 cells treated with transgenic aloe extracts for 6 h induced expression of the interferon stimulated gene 54, indicating activation of the IFN signaling pathway. The biological activity of the aloe produced IFNα2 was assessed using an antiviral assay with A549 cells treated with extracts from both the rind and pulp fractions of the shoot and subsequently infected with the lytic encephalomyocarditis virus. The highest level of activity attributable to recombinant IFNα2 was determined to be 625 IU/mg of total soluble protein (TSP) in the rind and 2,108 IU/mg TSP in the pulp. Two daughter plants that vegetatively budded during the course of this study were also confirmed to express IFNα2. These results confirm that Aloe vera is capable of expressing a human protein with biological activity, and that a secreted protein targeting the apoplast can be detected in the pulp fraction of the plant.

Mind bomb is a ubiquitin ligase that is essential for efficient activation of Notch signaling by Delta.

Lateral inhibition, mediated by Notch signaling, leads to the selection of cells that are permitted to become neurons within domains defined by proneural gene expression. Reduced lateral inhibition in zebrafish mib mutant embryos permits too many neural progenitors to differentiate as neurons. Positional cloning of mib revealed that it is a gene in the Notch pathway that encodes a RING ubiquitin ligase. Mib interacts with the intracellular domain of Delta to promote its ubiquitylation and internalization. Cell transplantation studies suggest that mib function is essential in the signaling cell for efficient activation of Notch in neighboring cells. These observations support a model for Notch activation where the Delta-Notch interaction is followed by endocytosis of Delta and transendocytosis of the Notch extracellular domain by the signaling cell. This facilitates intramembranous cleavage of the remaining Notch receptor, release of the Notch intracellular fragment, and activation of target genes in neighboring cells.

Inhibitors of ubiquitin-activating enzyme (E1), a new class of potential cancer therapeutics.

The conjugation of proteins with ubiquitin plays numerous regulatory roles through both proteasomal-dependent and nonproteasomal-dependent functions. Alterations in ubiquitylation are observed in a wide range of pathologic conditions, including numerous malignancies. For this reason, there is great interest in targeting the ubiquitin-proteasome system in cancer. Several classes of proteasome inhibitors, which block degradation of ubiquitylated proteins, are widely used in research, and one, Bortezomib, is now in clinical use. Despite the well-defined and central role of the ubiquitin-activating enzyme (E1), no cell permeable inhibitors of E1 have been identified. Such inhibitors should, in principle, block all functions of ubiquitylation. We now report 4[4-(5-nitro-furan-2-ylmethylene)-3,5-dioxo-pyrazolidin-1-yl]-benzoic acid ethyl ester (PYR-41) as the first such inhibitor. Unexpectedly, in addition to blocking ubiquitylation, PYR-41 increased total sumoylation in cells. The molecular basis for this is unknown; however, increased sumoylation was also observed in cells harboring temperature-sensitive E1. Functionally, PYR-41 attenuates cytokine-mediated nuclear factor-kappaB activation. This correlates with inhibition of nonproteasomal (Lys-63) ubiquitylation of TRAF6, which is essential to IkappaB kinase activation. PYR-41 also prevents the downstream ubiquitylation and proteasomal degradation of IkappaBalpha. Furthermore, PYR-41 inhibits degradation of p53 and activates the transcriptional activity of this tumor suppressor. Consistent with this, it differentially kills transformed p53-expressing cells. Thus, PYR-41 and related pyrazones provide proof of principle for the capacity to differentially kill transformed cells, suggesting the potential for E1 inhibitors as therapeutics in cancer. These inhibitors can also be valuable tools for studying ubiquitylation.

Expression, purification, and properties of the Ubc4/5 family of E2 enzymes.

Ubiquitin-conjugating enzymes (E2s) play a central role in ubiquitylation. They function to bridge the first, nonspecific step of ubiquitin activation by E1 with the transfer of activated ubiquitin to substrates by substrate-specific E3s. While sharing a common core UBC domain, members of this family exhibit significant specificity in their physical and functional interactions with E3s. Among the families of E2s, members of the yeast Ubc4/5 family are particularly well conserved in higher metazoans. In humans, these are represented by the UbcH5 family. Members of this ubiquitously expressed family show a capacity to interact with a wide range of E3s from both HECT and RING finger families, making them particularly useful tools in the laboratory. Using the UbcH5 family as a prototype, this chapter describes methods for the expression, purification, and characterization of E2 enzymes in vitro and some of the basics for their use in experiments in cells.

Expression and evaluation of RING finger proteins.

RING finger proteins represent the largest class of potential ubiquitin ligases. This chapter describes methods used to express and assess the activity of proteins containing RING fingers based on our experience with a number of different family members. In addition to general protocols for assessing activity, specific protocols are provided for evaluating the ubiquitylation of p53 by the RING finger E3 Hdm2/Mdm2. Use of these methods may help identify new E3s, dissect factors involved in ubiquitylation of substrates, and screen for molecules that affect ubiquitylation.

Studies of the ubiquitin proteasome system.

A concept that has arisen over the last decade is that proteins can, in general, be covalently modified by polypeptides, resulting in alterations in their fate and function. The first-identified and most well studied of these modifying polypeptides is ubiquitin. Although targeting for proteasomal degradation is the best studied outcome of ubiquitylation, we now understand that modification of proteins with ubiquitin has numerous other cellular roles that alter protein function and that are unrelated to proteasomal degradation. Ubiquitylation is a complex process that is regulated at the level of both addition and removal of ubiquitin from target proteins. This unit includes a number of different basic protocols that will facilitate the study of components of the ubiquitin system and substrate ubiquitylation both in vitro and in cells. Because another protein modifier, NEDD8, itself regulates aspects of the ubiquitin system, basic protocols on neddylation are also included in this unit.

E6AP mediates regulated proteasomal degradation of the nuclear receptor coactivator amplified in breast cancer 1 in immortalized cells.

The steroid receptor coactivator oncogene, amplified in breast cancer 1 (AIB1; also known as ACTR/RAC-3/TRAM-1/SRC-3/p/CIP), is amplified and overexpressed in a variety of epithelial tumors. AIB1 has been reported to have roles in both steroid-dependent and steroid-independent transcription during tumor progression. In this report, we describe that the cellular levels of AIB1 are controlled through regulated proteasomal degradation. We found that serum withdrawal or growth in high cell density caused rapid degradation of AIB1 protein, but not mRNA, in immortalized cell lines. Proteasome inhibitors prevented this process, and high molecular weight ubiquitylated species of AIB1 were detected. Nuclear export was required for proteasomal degradation of AIB1 and involved the ubiquitin ligase, E6AP. AIB1/E6AP complexes were detected in cellular extracts, and reduction of cellular E6AP levels with E6AP short interfering RNA prevented proteasomal degradation of AIB1. Conversely, overexpression of E6AP promoted AIB1 degradation. The COOH terminus of AIB1 interacted with E6AP in vitro and deletion of this region in AIB1 rendered it resistant to degradation in cells. From our results, we propose a model whereby signals promoted by changes in the cellular milieu initiate E6AP-mediated proteasomal degradation of AIB1 and thus contribute to the control of steady-state levels of this protein.

Multiple roles of Rbx1 in the VBC-Cul2 ubiquitin ligase complex.

The importance of the ubiquitin system largely depends on ubiquitin ligases, E3s, as they determine the specificity of the system. Rbx1/ROC1/Hrt1, a RING finger protein, functions as an important component of the cullin-containing SCF and VBC-Cul2 ligases. Modification of cullins by NEDD8 (NEDDylation), has been shown to be essential for the E3 activity of both SCF and VBC-Cul2, and it was suggested that Rbx1 acts as the E3 for cullin NEDDylation. RING finger is composed of eight cysteine and histidine residues that bind to zinc ions. Rbx1 is a highly evolutionarily conserved protein; however, the eighth coordination residue in its RING finger is aspartate (D97) rather than cysteine. Substitution of D97 with each of the other 19 amino acids demonstrates that aspartate is superior to cysteine in cullin NEDDylation. Interestingly, however, different D97 mutants demonstrate different activities towards 6 cullins tested. Importantly, we were able to discriminate between the NEDDylating activity of Rbx1 and its involvement in the ubiquitylation reaction within the context of VBC-Cul2. Moreover, while Rbx1 is not involved in governing the stability of SCF, Rbx1 mutants destabilize VBC-Cul2. Taken together, these results indicate that various mechanisms regulate both the activities and the stability of cullin-based ligases.

RING finger ubiquitin protein ligases: implications for tumorigenesis, metastasis and for molecular targets in cancer.

Covalent modification of proteins with ubiquitin regulates almost all aspects of eukaryotic cellular function. Ubiquitin protein ligases (E3s) play central regulatory roles in that they provide substrate specificity to this process and therefore, represent attractive molecular targets for disease therapy. We summarize recent advances in our understanding of RING finger and RING finger-related E3s with emphasis on BRCA1 and the tumor autocrine motility factor receptor (gp78), as well as discuss the potential for components of the ubiquitin pathway for proteasomal degradation as molecular targets.