Antibodies used are indicated in the left. and prevents diseases such as malignancy. This requires that this Resiniferatoxin genome is Eptifibatide Acetate usually faithfully replicated in a DNA synthesis (S) phase and each of the two resulting sets of sister chromatids are condensed and segregated properly to the two daughter cells during mitosis (M phase) (1). These cell-cycle events are tightly controlled and necessitate the concerted activity and timely regulation of a cohort of enzymes, including those that directly regulate the dynamic changes in chromatin structure critical for DNA replication, Resiniferatoxin chromosome compaction and cell division (2). A well-known example is the sense of balance exerted by the opposing action of histone H4 acetyltransferases (HAT) and deacetylases (HDAC) that modulates the levels of lysine acetylation on histone H4 and thus contributes to proper chromatin compaction during the cell cycle (3). Indeed, histone H4 acetylation is known to favor a more relaxed chromatin organization that is conducive to proper DNA replication initiation and S-phase progression (4). However, the mechanisms coordinating the activity of HAT and HDAC on histone H4 tail with the entry into S-phase still remain poorly comprehended. The SET-domain methyltransferase PR-Set7 (also known as SET8, SETD8 or KMT5A) is usually another histone H4 modifying enzyme responsible for the monomethylation of histone H4 at lysine 20 (H4K20me1) and of several other non-histone substrates (5,6). In mammalian cells, loss and gain of function studies show that PR-Set7 is essential for the maintenance of genome stability, which involves the timely destruction of the enzyme Resiniferatoxin during S-phase (7,8). This is Resiniferatoxin mediated by ubiquitin-mediated proteolysis and requires the interaction of the enzyme with the DNA replication factor PCNA through a conserved PCNA-interacting (PIP) motif located upstream of the catalytic SET domain name (9,10). PCNA serves as a cofactor to promote PR-Set7 interaction with the CRL4CDT2 E3 ubiquitin ligase, which earmarks PR-Set7 for ubiquitylation and degradation during S phase or upon DNA damage (10C14). PCNA-mediated degradation of mammalian PR-Set7 is essential for proper cell-cycle progression (14,15). Indeed, the mutation of the PIP-motif is sufficient to stabilize the enzyme and induces changes in chromatin compaction and DNA re-replication, which is usually partially due to the ability of PR-Set7 to stimulate the recruitment of pre-replication complex components on chromatin (13,16). In addition to the CRL4cdt2 pathway, the APCCdh1 and the F-box proteins Skp2 and -TRCP of SCF ubiquitin E3 ligase complexes have also been reported to regulate PR-Set7 stability in human cells (15,17C19). However, because of the dominant effect of CRL4cdt2 pathway on PR-Set7 stability, it remains largely unclear whether these additional PR-Set7 degradation pathways play a critical role in PR-Set7 functions or whether they serve as fine-tuning system to regulate the abundance of the enzyme in different phases of the cell cycle. Here, we have studied the functions of the ortholog of PR-Set7 (20). As its mammalian counterpart, we show that PR-Set7 is also subject to a proteolytic regulation during the cell cycle with the lowest levels from G1 to early S-phase. However, in contrast to mammals, a mutated PIP-motif neither stabilized PR-Set7 nor was critical for its functions in cell-cycle regulation during development. Thanks to the identification of a minimal functional sequence of PR-Set7 for proper cell proliferation, we confirmed that this catalytic activity of PR-Set7 is required for G2/M transition and revealed that targeting of the nuclear pool of this enzyme by Slimb, the ortholog of -TRCP, is required for G1/S transition. Finally, we show that nuclear accumulation of PR-Set7 upon Slimb depletion led to abnormal chromatin compaction and DNA replication inhibition, thereby causing G1/S arrest. Strikingly, these phenotypes are driven by non-enzymatic PR-Set7 functions that negatively regulate the levels of histone H4 acetylation. Altogether, these results identify the Slimb-mediated degradation of PR-Set7 by itself as a new crucial cell-cycle regulatory mechanism that ensures proper chromatin structure from G1 to S phase progression. MATERIALS AND METHODS Cell culture, establishment of stable cell lines, synchronization and RNA interference S2 Resiniferatoxin (L2C4) or adherent S2R+ cells were produced in Drosophila Schneider’s medium (InVitrogen) supplemented with 10% bovine serum and antibiotics (penicilline/ streptomycine). Cells were transfected according to manufacturer’s instructions using Effectene transfection agent (Qiagen). Stable cell lines were selected with 200 g/ml Hygromycin-B or 100 g/ml of Blasticidine. RNA interference in cells was conducted as previously described. Briefly, double stranded RNA (dsRNA) was produced using T7 RNA production kit (Promega) from PCR products generated with.