Following transfection, cells were synchronized by nocodazole treatment with a subsequent release from the mitotic block

Following transfection, cells were synchronized by nocodazole treatment with a subsequent release from the mitotic block. essential for regulating cell cycle progression[1],[2]. Activation of the APC/C is dependent on mitosis-specific phosphorylation of several subunits[3][6]and on the sequential binding of the WD-repeat-containing proteins, Cdc20 and Cdh1[7],[8], which are thought to recruit substrates to the core enzyme. Targeting of substrates by the APC/C depends on short destruction motifs in their primary sequence. The most commonly found sequences are the destruction-box (D-box, RxxL)[9],[10]and the KEN-box (KENxxxN)[11]. However, recent reports have revealed several other motifs apart from these canonical sites[12][15]. NIPA (nuclear interaction partner of ALK) was originally identified by our group as a human nuclear protein in a screen for interaction partners of the activated anaplastic lymphoma kinase (ALK) receptor tyrosine kinase[16]. We subsequently characterized NIPA as an F-box like protein that defines a ubiquitin E3 ligase (SCFNIPA) which targets nuclear cyclin B1 for degradation and thereby contributes to the timing of mitotic entry. Intriguingly, phosphorylation of NIPA in late G2 phase leads to dissociation of NIPA from the SCF core complex, thus restricting activity of the SCFNIPAcomplex to interphase[17],[18]. Here, we report that phosphorylated NIPA is degraded at mitotic exit NVP-231 in an APC/CCdh1-dependent manner. This degradation is regulated by the cell-cycle-dependent binding of NIPA to the SCF core-protein Skp1 and represents a novel mode of regulating APC/C-mediated ubiquitination. == Results == == The phosphorylated form of NIPA is degraded in late mitosis == Previous studies revealed phosphorylation of NIPA starting in late G2 phase of the cell cycle and peaking at the G2/M boundary (Fig. S1and ref.[17]). After the G2/M transition, NIPA phosphorylation and expression levels decline precipitously upon entry into G1 and an NVP-231 unphosphorylated form of NIPA reappears later in G1 (Fig. 1A, lanes 15). Treatment of the cells with the translation inhibitor cycloheximide (CHX) after release from prometaphase prevented accumulation of the non-phosphorylated form of NIPA in G1 NVP-231 (Fig. 1A, lanes 610). This result indicates that the appearance of NVP-231 the lower form of NIPA is due to new protein synthesis rather than dephosphorylation of the upper form of NIPA and thus suggests that the phosphorylated form of NIPA is degraded in late mitosis. Remarkably, phosphorylated NIPA was degraded simultaneously with Cdc20, Cyclin B1 and Cyclin A, three known mitotic substrates of the APC/C (Fig. 1B). == Figure 1. Phosphorylated NIPA is degraded during mitotic exit. == (A) NIH3T3 cells stably overexpressing Flag-NIPA were arrested in prometaphase by a NMA thymidine-nocodazole block and subsequently released into fresh medium either containing cycloheximide (CHX) or without supplements. Cells were collected at the indicated timepoints. The degree of synchronization was confirmed by analysis of the expression of cyclin B1 and FACS analysis (data not shown). (B) HeLa cells were arrested in NVP-231 prometaphase, released into fresh growth medium, and harvested for Western blot at the indicated timepoints. (C) HEK293T cells were transfected with (+) or without (-) HA-Ubiquitin and Flag-NIPA as indicated. Following treatment with MG132, extracts were prepared, denatured and subjected to Flag immunoprecipitation. Ubn: polyubiquitinated forms. (D) NIH3T3 cells retrovirally infected with a Flag-NIPA construct were treated with cycloheximide (CHX) and either DMSO or the proteasome inhibitor MG132. Cells were harvested for Western blot at the indicated timepoints. To investigate whether NIPA degradation may be regulated by ubiquitination, we tested whether NIPA is polyubiquitinatedin vivo. Therefore, cells were transfected with Flag-NIPA, HA-ubiquitin or with both. The transfected cells were treated with MG132 prior to harvesting. Immunoblotting detected high molecular weight ubiquitin conjugates in the Flag-NIPA immuno-complex in the presence of MG132 (Fig. 1C). To determine whether proteasomal function is required for NIPA degradation, NIPA-overexpressing cells were treated with cyclohexamide and the proteasome inhibitor MG132. Proteasome inhibition resulted in significant stabilization of NIPA (Fig. 1D). These results provide evidence that the ubiquitin-proteasome pathway controls the destruction of NIPA. == NIPA is a substrate of APC/CCdh1 == Since NIPA degradation occurs simultaneously with other APC/C-targets, we examined whether APC/C is required for NIPA degradation. To this end, we prepared extracts with high APC/C-activity and depleted APC/C from the extracts with antibody against Cdc27, a core subunit of APC/C, prior toin vitrodegradation assays. We observed that exogenous,.