FEM1 proteins are ancient regulators of SLBP degradation

<p>FEM1A, FEM1B, and FEM1C are evolutionarily-conserved VHL-box proteins, the substrate recognition subunits of CUL2-RING E3 ubiquitin ligase complexes. Here, we report that FEM1 proteins are ancient regulators of Stem-Loop Binding Protein (SLBP), a conserved protein that interacts with the stem loop structure located in the 3′ end of canonical histone mRNAs and functions in mRNA cleavage, translation and degradation. SLBP levels are highest during S-phase coinciding with histone synthesis. The ubiquitin ligase complex SCF^{cyclin F} targets SLBP for degradation in G2 phase; however, the regulation of SLBP during other stages of the cell cycle is poorly understood. We provide evidence that FEM1A, FEM1B, and FEM1C interact with and mediate the degradation of SLBP. Cyclin F, FEM1A, FEM1B and FEM1C all interact with a region in SLBP's N-terminus using distinct degrons. An SLBP mutant that is unable to interact with all 4 ligases is expressed at higher levels than wild type SLBP and does not oscillate during the cell cycle. We demonstrate that orthologues of SLBP and FEM1 proteins interact in <i>C. elegans</i> and <i>D. melanogaster</i>, suggesting that the pathway is evolutionarily conserved. Furthermore, we show that FEM1 depletion in <i>C. elegans</i> results in the upregulation of SLBP ortholog CDL-1 in oocytes. Notably, cyclin F is absent in flies and worms, suggesting that FEM1 proteins play an important role in SLBP targeting in lower eukaryotes.</p

(A) PAE cells stably expressing GFP-Rac1 at levels below endogenous Rac1 (Fig

S1, available at ) were scored for the percentage of cells showing strong nuclear fluorescence before and 16 h after the addition of increasing amounts of the indicated compounds. Representative cells at the indicated dose are shown with the percentage of cells showing each phenotype indicated (left), and cumulative dose-response data are shown on the right (mean ± SEM; = 3). Bars, 10 μm. (B) Endogenous Rac1, Ras, and RCC1 were measured in the nuclear fractions as described in before and after the addition of 50 μM apigenin for 24 h (mean ± SEM; = 4).<p><b>Copyright information:</b></p><p>Taken from "Rac1 accumulates in the nucleus during the G2 phase of the cell cycle and promotes cell division"</p><p></p><p>The Journal of Cell Biology 2008;181(3):485-496.</p><p>Published online 5 May 2008</p><p>PMCID:PMC2364699.</p><p></p

COS-1 cells were separated into nuclear and nonnuclear fractions as described in Materials and methods

Equal cell equivalents of each fraction were analyzed by SDS-PAGE and immunoblots (inset) for the indicated proteins. Immunoprecipitated proteins were detected and quantified with [I]protein A and phosphorimaging, and the percentage of total protein in the nuclear fraction was calculated (mean ± SEM; = 3).<p><b>Copyright information:</b></p><p>Taken from "Rac1 accumulates in the nucleus during the G2 phase of the cell cycle and promotes cell division"</p><p></p><p>The Journal of Cell Biology 2008;181(3):485-496.</p><p>Published online 5 May 2008</p><p>PMCID:PMC2364699.</p><p></p

PAE (A) or NIH 3T3 (B) cells stably expressing GFP-Rac1 at levels below endogenous were examined by time-lapse confocal microscopy over one division cycle

Arrowheads indicate representative parent and daughter cells. Note that nuclear Rac1 is high immediately preceding mitosis and that GFP-Rac1 is excluded from the nuclei of the daughter cells immediately after cell division. See Videos 1 and 2 (available at ). Bars, 10 μm.<p><b>Copyright information:</b></p><p>Taken from "Rac1 accumulates in the nucleus during the G2 phase of the cell cycle and promotes cell division"</p><p></p><p>The Journal of Cell Biology 2008;181(3):485-496.</p><p>Published online 5 May 2008</p><p>PMCID:PMC2364699.</p><p></p

(A) PAE cells stably expressing GFP-Rac1 at levels below endogenous were synchronized in G1/S by serum starvation followed by hydroxyurea and then released

The percentage of cells with nuclear Rac1 was determined hourly and plotted as mean ± SEM ( = 4). (B) Aliquots of the cells analyzed in A were scraped from plates at the indicated times and analyzed for stage of the cell cycle by propidium iodide and cytofluorimetry. (C) Unsynchronized COS-1 cells were transfected with GFP-Rac1 and, after 16 h, fixed and stained for cyclin A. Although the cell expressing GFP-Rac1 in the nucleus (arrows) stained for cyclin A, a marker of G2/M, those excluding the protein from the nucleus (arrowheads) did not. This correlation held for 93% of transfected cells examined (>100). Bars, 10 μm.<p><b>Copyright information:</b></p><p>Taken from "Rac1 accumulates in the nucleus during the G2 phase of the cell cycle and promotes cell division"</p><p></p><p>The Journal of Cell Biology 2008;181(3):485-496.</p><p>Published online 5 May 2008</p><p>PMCID:PMC2364699.</p><p></p

(A) Nuclear and nonnuclear fractions were prepared as described in Materials and methods

After separation, each fraction was brought to 1% Triton X-114 and phase separation was initiated by heating the samples to 37°C. The aqueous and detergent phases of each fraction were analyzed for Rac1, RhoGDI, and lamin B by immunoblotting. Immunoblots were quantified with [I]protein A and phosphorimaging, and the percentage of total protein in each fraction in the detergent phase was calculated (mean ± SEM; = 3). (B) Triton X-114 partition as shown in A was performed on the nuclear fractions of COS-1 cells treated overnight with or without 10 μM simvistatin. Aq, aqueous; Det, detergent phases. (C) Selected images of GFP-Rac1 in COS-1 and ECV cells showing prominent decoration of the nuclear envelope (arrowhead). Bars, 10 μm.<p><b>Copyright information:</b></p><p>Taken from "Rac1 accumulates in the nucleus during the G2 phase of the cell cycle and promotes cell division"</p><p></p><p>The Journal of Cell Biology 2008;181(3):485-496.</p><p>Published online 5 May 2008</p><p>PMCID:PMC2364699.</p><p></p

(A) Confocal images of asynchronous T98G cells expressing GFP-Rac1 with the percent of the transfected population represented by each pattern indicated

T98G (B) or IMR-90 (C) cells were synchronized by serum deprivation for 72 h and then induced to cycle by refeeding with 10% FBS. Aliquots of cells were harvested at the times indicated and assayed for Rac1 and the indicated control proteins by immunoblotting. Bar, 5 μm.<p><b>Copyright information:</b></p><p>Taken from "Rac1 accumulates in the nucleus during the G2 phase of the cell cycle and promotes cell division"</p><p></p><p>The Journal of Cell Biology 2008;181(3):485-496.</p><p>Published online 5 May 2008</p><p>PMCID:PMC2364699.</p><p></p