The RAD6 and RAD18 genes of Saccharomyces cerevisiae are required for postreplicative bypass of ultraviolet (UV)-damaged DNA and for UV mutagenesis. The RAD6 encoded protein is a ubiquitin conjugating enzyme, and RAD18 encodes a protein containing a RING finger motif and a nucleotide binding motif. Rad18 can be co-immunoprecipitated with Rad6, indicating that the two proteins exist in a complex in vivo. Here, we co-overproduce the two proteins using a yeast multicopy plasmid, purify the Rad6-Rad18 complex to near homogeneity, and show that the complex is heterodimeric. The Rad6-Rad18 heterodimer has ubiquitin conjugating activity, binds single-stranded DNA, and possesses single-stranded DNA-dependent ATPase activity. The Rad6-Rad18 complex provides the first example wherein a ubiquitin conjugating activity is physically associated with DNA binding and ATPase activities provided by an associated protein factor. The co-existence of these activities should provide the complex with the ability to recognize single-stranded DNA resulting from stalling of the replication machinery at DNA damage sites and to recognize the components of the DNA replication machinery for ubiquitination by Rad6.Exposure of cells to ultraviolet (UV) light and to many other agents causes the formation of lesions in the DNA. During DNA replication, such lesions located in the template strand block the DNA replication machinery, resulting in a gap in the newly synthesized strand across from the damage site. A variety of postreplicational repair mechanisms have evolved to restore the continuity of the newly synthesized DNA strand (reviewed in Ref. 1).Genetic studies in the yeast Saccharomyces cerevisiae have been instrumental in identifying the genes involved in postreplicational repair. RAD6 and RAD18, members of the RAD6 epistasis group, play a prominent role in this repair process. Mutations in RAD6 cause extreme sensitivity to UV light and to other DNA damaging agents; rad6 mutants are highly deficient in postreplicational repair of UV-damaged DNA (2) and they exhibit no mutation induction in response to UV (3). RAD6encodes an ubiquitin conjugating enzyme of 172 residues (4, 5). The first 149 amino acids of Rad6 form a globular domain, while the distal 23 residues, which are predominantly acidic, constitute a freely extending tail domain (6). Mutational inactivation of the active site cysteine 88 residue in Rad6 has indicated that the ubiquitin conjugating activity is essential for all the biological functions of Rad6 (7). Mutants of RAD18 resemble those of RAD6 in their high degree of sensitivity to UV, defects in postreplicational repair of UV-damaged DNA (2), and defects in UV mutagenesis (8, 9). However, unlike RAD6, which is indispensable for sporulation, mutations in RAD18 do not affect sporulation (10).Other genes that belong to the RAD6 epistasis group include REV1, REV3, REV7, and RAD5. Although mutants of the REV genes show only a marginal increase in UV sensitivity, like rad6 and rad18 mutants, they are defective in UV mutagenesi...

Homologous recombination is a fundamental biological process. Biochemical understanding of this process is most advanced for Escherichia coli. At least 25 gene products are involved in promoting genetic exchange. At present, this includes the RecA, RecBCD (exonuclease V), RecE (exonuclease VIII), RecF, RecG, RecJ, RecN, RecOR, RecQ, RecT, RuvAB, RuvC, SbcCD, and SSB proteins, as well as DNA polymerase I, DNA gyrase, DNA topoisomerase I, DNA ligase, and DNA helicases. The activities displayed by these enzymes include homologous DNA pairing and strand exchange, helicase, branch migration, Holliday junction binding and cleavage, nuclease, ATPase, topoisomerase, DNA binding, ATP binding, polymerase, and ligase, and, collectively, they define biochemical events that are essential for efficient recombination. In addition to these needed proteins, a cis-acting recombination hot spot known as Chi (chi: 5'-GCTGGTGG-3') plays a crucial regulatory function. The biochemical steps that comprise homologous recombination can be formally divided into four parts: (i) processing of DNA molecules into suitable recombination substrates, (ii) homologous pairing of the DNA partners and the exchange of DNA strands, (iii) extension of the nascent DNA heteroduplex; and (iv) resolution of the resulting crossover structure. This review focuses on the biochemical mechanisms underlying these steps, with particular emphases on the activities of the proteins involved and on the integration of these activities into likely biochemical pathways for recombination.

Genetic and biochemical studies of Saccharomyces cerevisiae have indicated the involvement of a large number of protein factors in nucleotide excision repair (NER) of UV-damaged DNA. However, how MMS19 affects this process has remained unclear. Here, we report on the isolation of the MMS19 gene and the determination of its role in NER and other cellular processes. Genetic and biochemical evidence indicates that besides its function in NER, MMS19 also affects RNA polymerase II (Pol II) transcription. mms19delta cells do not grow at 37 degrees C, and mutant extract exhibits a thermolabile defect in Pol II transcription. Thus, Mms19 protein resembles TFIIH in that it is required for both transcription and DNA repair. However, addition of purified Mms19 protein does not alleviate the transcriptional defect of the mms19delta extract, nor does it stimulate the incision of UV-damaged DNA reconstituted from purified proteins. Interestingly, addition of purified TFIIH corrects the transcriptional defect of the mms19delta extract. Mms19 is, however, not a component of TFIIH or of Pol II holoenzyme. These and other results suggest that Mms19 affects NER and transcription by influencing the activity of TFIIH as an upstream regulatory element. It is proposed that mutations in the human MMS19 counterpart could result in syndromes in which both NER and transcription are affected.

IL-12 is a cytokine which showed anti-tumor effects in clinical trials, but also produced serious toxicity. We describe a fusion protein, huBC1-IL12, designed to achieve an improved therapeutic index by specifically targeting IL-12 to tumor and tumor vasculature. huBC-1 is a humanized antibody that targets a cryptic sequence of the human ED-B-containing fibronectin isoform, B-FN, present in the subendothelial extracellular matrix of most aggressive tumors. B-FN is oncofetal and angiogenesis-associated, and is undetectable in most normal adult tissues. The original murine BC-1 antibody has been used successfully for immunoscintigraphy to image brain tumor mass in glioblastoma patients. In huBC1-IL12, each of the IgG heavy chains is genetically fused to the N-terminus of the IL-12 p35 subunit, which in turn is disulfide-bonded to the p40 subunit, resulting in a hexameric molecule of MW of approximately 300 kDa. Since human IL-12 has no biological activity in mice, we produced huBC1-muIL12 as a surrogate molecule for animal tumor models. Despite the relatively poor PK profile of this molecule in mice and the apparent drawbacks of xenogeneic models in SCID mice, which lack T and B cells, one cycle of treatment with huBC1-muIL12 was efficacious in the PC3mm2, A431, and HT29 subcutaneous tumor models and PC3mm2 lung metastasis model. This molecule also was found to have surprisingly low toxicity in immunocompetent mice. A fusion protein that contains human IL-12 (huBC1-huIL12), which is a suitable molecule for investigation as a therapeutic, has also been produced. This protein has been shown to have a longer serum half-life than huBC1-muIL12 in mice, and retains both antigen binding and IL-12 activity in in vitro assays.

Erythropoietin (Epo) is a cytokine that controls the production of red blood cells (RBCs). Epo acts continuously on RBC precursors to prevent apoptosis, so it is important to maintain high levels of Epo activity when treating anemic patients. We describe here modified human Epo [Epo(NDS)] with mutations His32Gly, Cys33Pro, Trp88Cys and Pro90Ala that result in the rearrangement of the disulfide bonding pattern from Cys29-Cys33 to Cys29-Cys88 and that, in the context of an Fc-Epo(NDS) fusion protein, lead to significantly improved properties. Fc-Epo was secreted from NS/0 myeloma cells as about 35% high molecular weight aggregates, was unstable upon removal of N-linked oligosaccharides and showed poor pharmacokinetics and little efficacy in mice. In contrast, a corresponding Fc-Epo(NDS) was secreted almost exclusively as a unit dimer, was relatively stable to removal of N-linked oligosaccharides, had much improved pharmacokinetic properties and had a significantly improved effect on RBC production. These results indicate that rearrangement of the disulfide bonding pattern in a therapeutic protein can have a significant effect on pharmacokinetics and, potentially, the dosing schedule of a protein drug.