Human hRad1 but not hRad9 protects hHus1 from ubiquitin–proteasomal degradation

Erp57 facilitates ZIKV-induced DNA damage via NS2B/NS3 complex formation

It is believed that DNA double-strand breaks induced by Zika virus (ZIKV) infection in pregnant women is a main reason of brain damage (e.g. microcephaly, severe brain malformation, and neuropathy) in newborn babies [1,2], but its underlying mechanism is poorly understood. In this study, we report that the depletion of ERp57, a member of the protein disulphide isomerase (PDI) family, leads to the limited production of ZIKV in nerve cells. ERp57 knockout not only suppresses viral induced reactive oxygen species (ROS) mediated host DNA damage, but also decreases apoptosis. Strikingly, DNA damage depends on ERp57-bridged complex formation of viral protein NS2B/NS3. LOC14, an ERp57 inhibitor, restricts ZIKV infection and virus-induced DNA damage. Our work reveals an important role of ERp57 in both ZIKV propagation and virus-induced DNA damage, suggesting a potential target against ZIKV infection.

The dynamic subcellular localisation of Rad1 is cell cycle dependent in Leishmania major

The subcellular localisation of Rad1, a subunit of the Leishmania major 9-1-1 complex, remains unexplored. Herein, we reveal that Rad1 localises predominantly to the nucleus. Upon hydroxyurea treatment, the diffuse nuclear localisation of Rad1 becomes more punctate, suggesting that Rad1 is responsive to replication stress. Moreover, Rad1 localisation correlates with cell cycle progression. In the majority of G1 to early S-phase cells, Rad1 localises predominantly to the nucleus. As cells progress from late-S phase to mitosis, Rad1 relocalizes to both the nucleus and the cytoplasm in ∼90 % of cells. This pattern of distribution is different from Rad9 and Hus1, which remain nuclear throughout the cell cycle, suggesting Leishmania Rad1 may regulate 9-1-1 activities and/or perform relevant functions outside the 9-1-1 complex.

The rad1 gene in Rainbow Trout (Oncorhynchus mykiss) is highly conserved and may express proteins from non-canonical spliced isoforms

Cell-cycle checkpoint proteins maintain genomic integrity by sensing damaged DNA and initiating DNA repair or apoptosis. RAD1 is a checkpoint protein involved in the sensing of damaged DNA and is a part of the 9-1-1 complex. In this project rainbow trout rad1 (rtrad1) was cloned, sequenced, expressed as a recombinant protein and anti-rtRAD1 antibodies were developed. RAD1 protein levels were characterized in various rainbow trout tissues. It was determined that an 840 bp open-reading frame encodes 279 aa with a predicted protein size of 31 kDa. The rtRAD1 amino-acid sequence is highly conserved and contains conserved exonuclease and leucine zipper domains. RT-PCR was used to identify three non-canonical splice variants of rtrad1, two of which are capable of forming functional proteins. The rad1 splice variant that encodes an 18 kDa protein appears to be abundant in rainbow trout spleen, heart and gill tissue and in the RTgill-W1 cell-line. Based on the genomic rtrad1 sequence the splice variants contain only partial exons which are consistent with the splicing of rad1 variants in mammals. This is the first time that rad1 has been fully characterized in a fish species.

Localization of the Drosophila Rad9 protein to the nuclear membrane is regulated by the C-terminal region and is affected in the meiotic checkpoint

Rad9, Rad1, and Hus1 (9-1-1) are part of the DNA integrity checkpoint control system. It was shown previously that the C-terminal end of the human Rad9 protein, which contains a nuclear localization sequence (NLS) nearby, is critical for the nuclear transport of Rad1 and Hus1. In this study, we show that in Drosophila, Hus1 is found in the cytoplasm, Rad1 is found throughout the entire cell and that Rad9 (DmRad9) is a nuclear protein. More specifically, DmRad9 exists in two alternatively spliced forms, DmRad9A and DmRad9B, where DmRad9B is localized at the cell nucleus, and DmRad9A is found on the nuclear membrane both in Drosophila tissues and also when expressed in mammalian cells. Whereas both alternatively spliced forms of DmRad9 contain a common NLS near the C terminus, the 32 C-terminal residues of DmRad9A, specific to this alternative splice form, are required for targeting the protein to the nuclear membrane. We further show that activation of a meiotic checkpoint by a DNA repair gene defect but not defects in the anchoring of meiotic chromosomes to the oocyte nuclear envelope upon ectopic expression of non-phosphorylatable Barrier to Autointegration Factor (BAF) dramatically affects DmRad9A localization. Thus, by studying the localization pattern of DmRad9, our study reveals that the DmRad9A C-terminal region targets the protein to the nuclear membrane, where it might play a role in response to the activation of the meiotic checkpoint.

Prefoldin subunits are protected from ubiquitin-proteasome system-mediated degradation by forming complex with other constituent subunits

The molecular chaperone prefoldin (PFD) is a complex comprised of six different subunits, PFD1-PFD6, and delivers newly synthesized unfolded proteins to cytosolic chaperonin TRiC/CCT to facilitate the folding of proteins. PFD subunits also have functions different from the function of the PFD complex. We previously identified MM-1α/PFD5 as a novel c-Myc-binding protein and found that MM-1α suppresses transformation activity of c-Myc. However, it remains unclear how cells regulate protein levels of individual subunits and what mechanisms alter the ratio of their activities between subunits and their complex. In this study, we found that knockdown of one subunit decreased protein levels of other subunits and that transfection of five subunits other than MM-1α into cells increased the level of endogenous MM-1α. We also found that treatment of cells with MG132, a proteasome inhibitor, increased the level of transfected/overexpressed MM-1α but not that of endogenous MM-1α, indicating that overexpressed MM-1α, but not endogenous MM-1α, was degraded by the ubiquitin proteasome system (UPS). Experiments using other PFD subunits showed that the UPS degraded a monomer of PFD subunits, though extents of degradation varied among subunits. Furthermore, the level of one subunit was increased after co-transfection with the respective subunit, indicating that there are specific combinations between subunits to be stabilized. These results suggest mutual regulation of protein levels among PFD subunits and show how individual subunits form the PFD complex without degradation.

Development of bimolecular fluorescence complementation using Dronpa for visualization of protein-protein interactions in cells

PURPOSE

We developed a bimolecular fluorescence complementation (BiFC) strategy using Dronpa, a new fluorescent protein with reversible photoswitching activity and fast responsibility to light, to monitor protein-protein interactions in cells.

PROCEDURES

Dronpa was split at residue Glu164 in order to generate two Dronpa fragments [Dronpa N-terminal: DN (Met1-Glu164), Dronpa C-terminal: DC (Gly165-Lys224)]. DN or DC was separately fused with C terminus of hHus1 or N terminus of hRad1. Flexible linker [(GGGGS)×2] was introduced to enhance Dronpa complementation by hHus1-hRad1 interaction. Furthermore, we developed expression vectors to visualize the interaction between hMYH and hHus1. Gene fragments corresponding to the coding regions of hMYH and hHus1 were N-terminally or C-terminally fused with DN and DC coding region.

RESULTS

Complemented Dronpa fluorescence was only observed in HEK293 cells cotransfected with hHus1-LDN and DCL-hRad1 expression vectors, but not with hHus1-LDN or DCL-hRad1 expression vector alone. Western blot analysis of immunoprecipitated samples using anti-c-myc or anti-flag showed that DN-fused hHus1 interacted with DC-fused hRad1. Complemented Dronpa fluorescence was also observed in cells cotransfected with hMYH-LDN and DCL-hHus1 expression vectors or hMYH-LDN and hHus1-LDC expression vectors. Furthermore, complemented Dronpa, induced by the interaction between hMYH-LDN and DCL-hHus1, showed almost identical photoswitching activity as that of native Dronpa.

CONCLUSION

These results demonstrate that BiFC using Dronpa can be successfully used to investigate protein-protein interaction in live cells. Furthermore, the fact that complemented Dronpa has a reversible photoswitching activity suggests that it can be used as a tool for tracking protein-protein interaction.

LYL1 degradation by the proteasome is directed by a N-terminal PEST rich site in a phosphorylation-independent manner

Background

The Lymphoblastic leukemia 1 (LYL1) gene is a proto-oncogenic transcription factor found upregulated in patients with T-cell acute lymphoblastic leukemia (T-cell ALL). Initially, the upregulation was described to be as a result of a translocation. However, further studies revealed that transcriptional upregulation of LYL1could also occur without translocations. In addition, post-translational mechanisms, such as protein degradation could influence LYL1 expression as well.

Methodology/Principal Findings

In this study, we considered possible post-translational regulation of Lyl1, and investigated fundamental mechanisms governing LYL1 degradation in cell-based culture assays. We identify a PEST sequence motif located in the N-terminus of LYL1, which determines the efficiency of LYL1 degradation by the proteasome. The absence of the PEST degradation site leads to accumulation or upregulation of LYL1. We also show that LYL1 is phosphorylated by MAPK at S₃₆, and determined that proteasomal degradation of LYL1 occurs in a phosphorylation-independent manner.

Conclusions/Significance

Understanding LYL1 degradation is a step forward not only towards deciphering the normal function and regulation of LYL1, but could suggest post-translational mechanisms for upregulation of LYL1 that may contribute to its oncogenic role.

Jab1 mediates protein degradation of the Rad9-Rad1-Hus1 checkpoint complex

The Rad1-Rad9-Hus1 (9-1-1) complex serves a dual role as a DNA-damage sensor in checkpoint signaling and as a mediator in the DNA repair pathway. However, the intercellular mechanisms that regulate the 9-1-1 complex are poorly understood. Jab1, the fifth component of the COP9 signalosome complex, has a central role in the degradation of multiple proteins and is emerging as an important regulator in cancer development. Here, we tested the hypothesis that Jab1 controls the protein stability of the 9-1-1 complex via the proteosome pathway. We provide evidence that Jab1 physically associates with the 9-1-1 complex, and show that this association is mediated through direct interaction between Jab1 and Rad1, one of the subunits of the 9-1-1 complex. Importantly, Jab1 causes translocation of the 9-1-1 complex from the nucleus to the cytoplasm, mediating rapid degradation of the 9-1-1 complex via the 26 S proteasome. Furthermore, Jab1 significantly suppresses checkpoint signaling activation, DNA synthesis recovery from blockage and cell viability after replication stresses such as UV exposure, gamma radiation and treatment with hydroxyurea. These results suggest that Jab1 is an important regulator for the stability of protein 9-1-1 control in cells, which may provide novel information on the involvement of Jab1 in the checkpoint and DNA repair signaling in response to DNA damage.

Fhit Modulates the DNA Damage Checkpoint Response

In preneoplastic lesions, the DNA damage checkpoint is induced and loss of heterozygosity at the FRA3B/FHIT common chromosome fragile region precedes or is coincident with activation of the checkpoint response in these early stages. Introduction of exogenous Fhit into cells in vitro led to modulation of expression of checkpoint proteins Hus1 and Chk1 at mid-S checkpoint, a modulation that led to induction of apoptosis in esophageal cancer cells but not in noncancerous primary cultures. Mutation of the conserved Fhit tyrosine 114 resulted in failure of this function, confirming the importance of this residue. The results suggest that the DNA damage-susceptible FRA3B/FHIT chromosome fragile region, paradoxically, encodes a protein that is necessary for protecting cells from accumulation of DNA damage through its role in modulation of checkpoint proteins, and inactivation of Fhit contributes to accumulation of abnormal checkpoint phenotypes in cancer development.

Lens epithelium-derived growth factor/p75 prevents proteasomal degradation of HIV-1 integrase

The transcriptional coactivator lens epithelium-derived growth factor (LEDGF)/p75 acts as a chromatin tethering factor for human immunodeficiency virus type 1 (HIV-1) integrase protein, determining its nuclear localization and its tight association with nuclear DNA. Here we identify a second function for the LEDGF/p75-integrase interaction. We observed that stable introduction of HIV-1 integrase (IN) transcription units into cells made stringently LEDGF/p75-deficient by RNAi resulted in much lower steady state levels of IN protein than introduction into LEDGF/p75 wild type cells. The same LEDGF/p75-dependent disparity was observed for feline immunodeficiency virus IN. However, IN mRNA levels were equivalent in the presence and absence of LEDGF/p75. A post-translational mechanism was confirmed when the half-life of HIV-1 IN protein was found to be much shorter in LEDGF/p75-deficient cells. Proteasome inhibition fully countered this extreme instability, increasing IN protein levels to those seen in LEDGF/p75 wild type cells and implicating proteasomal destruction as the main cause of IN instability. Consistent with these data, increased ubiquitinated HIV-1 IN was found in the LEDGF/p75 knock-down cells. Moreover, restoration of LEDGF/p75 to knocked down clones rescued HIV-1 IN stability. Subcellular fractionation showed that HIV-1 IN is exclusively cytoplasmic in LEDGF/p75-deficient cells, but mainly nuclear in LEDGF/p75 wild type cells, and that cytoplasmic HIV-1 IN has a shorter half-life than nuclear HIV-1 IN. However, using LEDGF proteins defective for nuclear localization and IN interaction, we further determined that protection of HIV-1 IN from the proteasome requires neither chromatin tethering nor nuclear residence. Protection requires only interaction with LEDGF/p75, and it is independent of the subcellular localization of the IN-LEDGF complex.