Since cdc48 mutants were isolated by the first genetic screens for cell division cycle (cdc) mutants in yeast, the requirement of the chaperone-like ATPase Cdc48/p97 during cell division has remained unclear. Here, we discover an unanticipated function for Caenorhabditis elegans CDC-48 in DNA replication linked to cell cycle control. Our analysis of the CDC-48 UFD؊1/NPL؊4 complex identified a general role in S phase progression of mitotic cells essential for embryonic cell division and germline development of adult worms. These developmental defects result from activation of the DNA replication checkpoint caused by replication stress. Similar to loss of replication licensing factors, DNA content is strongly reduced in worms depleted for CDC-48, UFD-1, and NPL-4. In addition, these worms show decreased DNA synthesis and hypersensitivity toward replication blocking agents. Our findings identified a role for CDC-48 UFD؊1/NPL؊4 in DNA replication, which is important for cell cycle progression and genome stability.ATL-1/ATR ͉ C. elegans ͉ CDC-48/p97 ͉ genome stability M any biological processes including development and cell division are tightly controlled by ubiquitin-mediated protein degradation. A central factor for mobilizing and targeting ubiquitylated substrates to the 26S proteasome is Cdc48/p97 (Cdc48 in yeast, CDC-48 in C. elegans, p97 in mammals), a chaperone-like AAA ATPase (1). Its activity is modulated by alternative adaptor proteins, which determine recruitment and processing of specific substrates. Cdc48/p97 forms a complex with the cofactors Ufd1 and Npl4 that is involved in endoplasmic reticulum (ER)-associated protein degradation (ERAD) (2), membrane fusion and cell cycle progression (3, 4). Temperature sensitive cdc48 mutants have already been isolated by early cdc-screens (5) in Saccharomyces cerevisiae. However, the essential role of Cdc48 during cell cycle progression remained elusive.Meanwhile, different activities of Cdc48/p97 in mitosis have been addressed by several studies. Early observations in yeast and recent findings using Xenopus egg extracts suggested that Cdc48/p97 regulates spindle disassembly during exit from mitosis (6, 7). For example, spindle regulators such as the Polo-like kinase Plx remain attached and probably stabilize the spindle in the absence of p97 Ufd1/Npl4 . However, contradictory evidence exists concerning a specific role of the p97 Ufd1/Npl4 complex in spindle dynamics (8,9). Beside spindle function, p97 together with its Ufd1-Npl4 cofactor is important for nuclear envelope assembly (10). Interestingly, it has been shown that p97 stimulates nucleus reformation after mitosis by extracting and thereby inactivating the mitotic progression kinase Aurora B from chromatin (11). Together, these diverse processes involving Cdc48/p97 suggest the existence of multiple substrates that need to be regulated during mitosis.Recently, we found that the C. elegans Cdc48/p97 homologues CDC-48.1 and CDC-48.2 form an evolutionarily conserved complex with UFD-1 and NPL-4 important for t...

Fibrodysplasia ossificans progressiva (FOP) is a disabling genetic disorder of progressive heterotopic ossification (HO). Here, we report a patient with an ultra-rare point mutation [c.619C>G, p.Q207E] located in a codon adjacent to the most common FOP mutation [c.617G>A, p.R206H] of Activin A Receptor, type 1 (ACVR1) and that affects the same intracellular amino acid position in the GS activation domain as the engineered constitutively active (c.a.) variant p.Q207D. It was predicted that both mutations at residue 207 have similar functional effects by introducing a negative charge. Transgenic p.Q207D-c.a. mice have served as a model for FOP HO in several in vivo studies. However, we found that the engineered ACVR1(Q207D-c.a.) is significantly more active than the classic FOP mutation ACVR1(R206H) when overexpressed in chicken limbs and in differentiation assays of chondrogenesis, osteogenesis and myogenesis. Importantly, our studies reveal that the ACVR1(Q207E) resembles the classic FOP receptor in these assays, not the engineered ACVR1(Q207D-c.a.). Notably, reporter gene assays revealed that both naturally occurring FOP receptors (ACVR1(R206H) and ACVR1(Q207E)) were activated by BMP7 and were sensitive to deletion of the ligand binding domain, whereas the engineered ACVR1(Q207D-c.a.) exhibited ligand independent activity. We performed an in silico analysis and propose a structural model for p.Q207D-c.a. that irreversibly relocates the GS domain into an activating position, where it becomes ligand independent. We conclude that the engineered p.Q207D-c.a. mutation has severe limitations as a model for FOP, whereas the naturally occurring mutations p.R206H and p.Q207E facilitate receptor activation, albeit in a reversible manner.

Patients with Fibrodysplasia Ossificans Progressiva (FOP) suffer from ectopic bone formation, which progresses during life and results in dramatic movement restrictions. Cause of the disease are point mutations in the Activin A receptor type 1 (ACVR1), with p.R206H being most common. In this study we compared the signalling responses of ACVR1 and ACVR1 to different ligands. ACVR1, but not ACVR1 inhibited BMP signalling of BMP2 or BMP4 in a ligand binding domain independent manner. Likewise, the basal BMP signalling activity of the receptor BMPR1A or BMPR1B was inhibited by ACVR1, but enhanced by ACVR1. In comparison, BMP6 or BMP7 activated ACVR1 and caused a hyper-activation of ACVR1. These effects were dependent on an intact ligand binding domain. Finally, the neofunction of Activin A in FOP was tested and found to depend on the ligand binding domain for activating ACVR1. We conclude that the FOP mutation ACVR1 is more sensitive to a number of natural ligands. The mutant receptor apparently lost some essential inhibitory interactions with its ligands and co-receptors, thereby conferring an enhanced ligand-dependent signalling and stimulating ectopic bone formation as observed in the patients.