Polycomb Requires Chaperonin Containing TCP-1 Subunit 7 for Maintaining Gene Silencing in Drosophila

Glucose-regulated protein 78 regulates the subunit-folding of the CCT complex by modulating gene expression and protein interaction in the microsporidian Nosema bombycis

Chaperonin containing tailless complex polypeptide 1 (CCT) functions as a molecular chaperone and is essential for ensuring proper protein folding. Glucose-regulated protein 78 (GRP78/Bip), also a type of chaperone, not only assists in folding of proteins, but also facilitates the transportation of proteins into the endoplasmic reticulum (ER) via the Sec protein complex. In this study, we identified the CCTη of N. bombycis (NbCCTη) for the first time. Immunoprecipitations and mass spectrometry (IP-MS) of NbCCTη analysis showed that NbBip may interact with CCT subunits. Yeast two-hybrid assays validated that NbBip interacts with NbCCTη, as well as NbCCTα and NbCCTε. Furthermore, RNA interference on NbBip brought about radical expression of NbCCTα, NbCCTε, and NbCCTη, while RNAi on NbCCT subunits resulted in abnormal expression of NbBip. Immunofluorescence assay results showed that NbBip colocalized with NbCCTα and NbCCTη, and CCTη colocalized with Nbβ-tubulin and Nbactin in the parasite. Collectively, these findings suggest that NbBip may act as a crucial factor in the subunit-folding and assembly of CCT complex in N. bombycis.

A genome-wide screen identifies SCAI as a modulator of the UV-induced replicative stress response

Helix-destabilizing DNA lesions induced by environmental mutagens such as UV light cause genomic instability by strongly blocking the progression of DNA replication forks (RFs). At blocked RF, single-stranded DNA (ssDNA) accumulates and is rapidly bound by Replication Protein A (RPA) complexes. Such stretches of RPA-ssDNA constitute platforms for recruitment/activation of critical factors that promote DNA synthesis restart. However, during periods of severe replicative stress, RPA availability may become limiting due to inordinate sequestration of this multifunctional complex on ssDNA, thereby negatively impacting multiple vital RPA-dependent processes. Here, we performed a genome-wide screen to identify factors that restrict the accumulation of RPA-ssDNA during UV-induced replicative stress. While this approach revealed some expected "hits" acting in pathways such as nucleotide excision repair, translesion DNA synthesis, and the intra-S phase checkpoint, it also identified SCAI, whose role in the replicative stress response was previously unappreciated. Upon UV exposure, SCAI knock-down caused elevated accumulation of RPA-ssDNA during S phase, accompanied by reduced cell survival and compromised RF progression. These effects were independent of the previously reported role of SCAI in 53BP1-dependent DNA double-strand break repair. We also found that SCAI is recruited to UV-damaged chromatin and that its depletion promotes nascent DNA degradation at stalled RF. Finally, we (i) provide evidence that EXO1 is the major nuclease underlying ssDNA formation and DNA replication defects in SCAI knockout cells and, consistent with this, (ii) demonstrate that SCAI inhibits EXO1 activity on a ssDNA gap in vitro. Taken together, our data establish SCAI as a novel regulator of the UV-induced replicative stress response in human cells.