[Show abstract][Hide abstract] ABSTRACT: The cellular response to double-strand breaks (DSBs) in DNA is a complex signalling network, mobilized by the nuclear protein kinase ataxia-telangiectasia mutated (ATM), which phosphorylates many factors in the various branches of this network. A main question is how ATM regulates DSB repair. Here, we identify the DNA repair enzyme polynucleotide kinase/phosphatase (PNKP) as an ATM target. PNKP phosphorylates 5'-OH and dephosphorylates 3'-phosphate DNA ends that are formed at DSB termini caused by DNA-damaging agents, thereby regenerating legitimate ends for further processing. We establish that the ATM phosphorylation targets on human PNKP-Ser 114 and Ser 126-are crucial for cellular survival following DSB induction and for effective DSB repair, being essential for damage-induced enhancement of the activity of PNKP and its proper accumulation at the sites of DNA damage. These findings show a direct functional link between ATM and the DSB-repair machinery.
[Show abstract][Hide abstract] ABSTRACT: Mutation of tubulin chaperone E (TBCE) underlies hypoparathyroidism, retardation, and dysmorphism (HRD) syndrome with defective microtubule (MT) cytoskeleton. TBCE/yeast Pac2 comprises CAP-Gly, LRR (leucine-rich region), and UbL (ubiquitin-like) domains. TBCE folds alpha-tubulin and promotes alpha/beta dimerization. We show that Pac2 functions in MT dynamics: the CAP-Gly domain binds alpha-tubulin and MTs, and functions in suppression of benomyl sensitivity of pac2Delta mutants. Pac2 binds proteasomes: the LRR binds Rpn1, and the UbL binds Rpn10; the latter interaction mediates Pac2 turnover. The UbL also binds the Skp1-Cdc53-F-box (SCF) ubiquitin ligase complex; these competing interactions for the UbL may impact on MT dynamics. pac2Delta mutants are sensitive to misfolded protein stress. This is suppressed by ectopic PAC2 with both the CAP-Gly and UbL domains being essential. We propose a novel role for Pac2 in the misfolded protein stress response based on its ability to interact with both the MT cytoskeleton and the proteasomes.
Full-text · Article · Mar 2010 · Cellular and Molecular Life Sciences CMLS
[Show abstract][Hide abstract] ABSTRACT: Exportin-5, an evolutionarily conserved nuclear export factor of the beta-karyopherin family, exports phosphorylated proteins and small noncoding RNAs. Msn5, the yeast ortholog, exports primarily phosphorylated cargoes including Ho endonuclease and a number of transcription factors and regulatory proteins. The Msn5-mediated nuclear export of Ho is dependent on phosphorylation of Thr225 by kinases of the DNA damage response pathway. Although Msn5 has been the object of many studies, no NES sequence capable of binding the exportin and/or of leading to Msn5-dependent export of a heterologous protein has been identified. Here we report identification of a 13-residue Ho sequence that interacts with Msn5 in vitro and directs Msn5-dependent nuclear export of GFP in vivo. A single point mutation in this 13-mer Ho NES abrogates both interaction with Msn5 and nuclear export of Ho and of GFP. However, this mutation, or of T225A, both of which abrogate nuclear export of Ho, does not interfere with its interaction with Msn5 implying that the exportin makes multiple contacts with its cargo. This can explain the lack of a conserved NES in Msn5 cargoes. Our results identify essential criteria for Msn5-mediated nuclear export of Ho: phosphorylation on HoT225, and interaction with the 13-mer Ho NES sequence.
[Show abstract][Hide abstract] ABSTRACT: The yeast F-box protein Ufo1 recruits proteins for ubiquitylation by the SCF ubiquitin ligase complex preparing them for proteasomal degradation. Ufo1 has a role in maintenance of genome stability; its substrates include Ho endonuclease and Rad30 polymerase of error-prone DNA repair. Ufo1 is an unusual F-box protein, as it has three ubiquitin interacting motifs (UIMs). Deletion of the genomic UIMs is lethal; ectopic expression of UFO1 Delta UIMs extends protein half-life and arrests the cell cycle. A whole-genome study employing a TAP tag fused to the C-terminal UIMs did not identify Ufo1-interacting proteins. Here we therefore used stabilized N-terminally tagged Ufo1 Delta UIM as a strategy to identify Ufo1-interacting proteins by mass spectroscopy. We identified proteins that function in transcription, and an indirect interaction with Hsp70 molecular chaperones via the Skp1 adaptor; we also show that Ufo1 interacts with the 19S regulatory particle of the proteasome. Thus, our data augment the current network of known Ufo1 interacting proteins. We show directly that the UIMs are crucial for Ufo1 ubiquitylation in vivo, indicating that they facilitate turnover of SCF Ufo1 complexes. This allows recycling of the core subunits of the SCF complex and cell cycle progression.