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The identification of genes that regulate proliferation is of great importance to developmental biology, regenerative medicine and cancer research. Using an in situ screen on a cortical structure of the medaka fish brain, we identified the simplet gene (smp), which is homologous to the human FAM53B gene. smp was expressed in actively proliferating cells of the CNS throughout embryogenesis. It belongs to a family of vertebrate-specific genes with no characterized biochemical domains. We showed that FAM53B bound 14-3-3 chaperones, as well as SKIIP proteins, adaptor proteins connecting DNA-binding proteins to modulators of transcription. smp inactivation with morpholinos led to delayed epiboly and reduced embryonic size. Absence of Smp activity did not induce apoptosis, but resulted in a reduced cell proliferation rate and enlarged blastomeres. Moreover, smp was shown to control the expression of the pluripotency-associated oct4/pou5f1 gene. We propose that smp is a novel vertebrate-specific gene needed for cell proliferation and that it is probably associated with the maintenance of a pluripotent state.

International audience; Cells respond to DNA or mitotic spindle damage by activating specific pathways that halt the cell cycle to allow for possible repair. Here, we report that inactivation of one of the Saccharomyces cerevisiae 14-3-3 proteins, Bmh1, as well as the bmh1-S189P bmh2 mutant, failed to exhibit normal spindle damage-induced cell cycle delay and conferred hypersensitivity to benomyl or nocodazole. These defects were additive with those conferred by the bub2 and mad2 spindle checkpoint mutations. Following cdc13-1-induced DNA damage, the 14-3-3 response was additive with those provided by the Mec1 (ATR-related)-controlled Rad53 (CHK2-related) and Chk1 (CHK1-related) checkpoint pathways and also distinct from the PKA (Protein Kinase A)-controlled response. Therefore, the budding yeast 14-3-3 proteins contribute to the robustness of the two major mitotic checkpoints and, by doing so, may also ensure optimal coordination between the responses to two distinct types of damage.

The role of the mitotic phosphorylation of the amino (NH 2 ) terminus of Centromere Protein A (CENP-A), the histone variant epigenetic centromeric marker, remains elusive. Here, we show that the NH 2 terminus of human CENP-A is essential for mitotic progression and that localization of CENP-C, another key centromeric protein, requires only phosphorylation of the CENP-A NH 2 terminus, and is independent of the CENP-A NH 2 terminus length and amino acid sequence. Mitotic CENP-A nucleosomal complexes contain CENP-C and phosphobinding 14-3-3 proteins. In contrast, mitotic nucleosomal complexes carrying nonphosphorylatable CENP-A–S7A contained only low levels of CENP-C and no detectable 14-3-3 proteins. Direct interactions between the phosphorylated form of CENP-A and 14-3-3 proteins as well as between 14-3-3 proteins and CENP-C were demonstrated. Taken together, our results reveal that 14-3-3 proteins could act as specific mitotic “bridges,” linking phosphorylated CENP-A and CENP-C, which are necessary for the platform function of CENP-A centromeric chromatin in the assembly and maintenance of active kinetochores.

Cell surface expression of transmembrane proteins is strictly regulated. Mutually exclusive interaction with COPI or 14-3-3 proteins has been proposed as a mechanism underlying such trafficking control of various proteins. In particular, 14-3-3 dimers have been proposed to “sense” correctly assembled oligomers, allowing their surface targeting by preventing COPI-mediated intracellular retention. Here we examined whether such a mechanism is involved in the quality control of the heterodimeric G protein-coupled GABAB receptor. Its GB1 subunit, carrying the retention signal RSR, only reaches the cell surface when associated with the GB2 subunit. We show that COPI and 14-3-3 specifically bind to the GB1 RSR sequence and that COPI is involved in its intracellular retention. However, we demonstrate that the interaction with 14-3-3 is not required for proper function of the GABAB receptor quality control. Accordingly, competition between 14-3-3 and COPI cannot be considered as a general trafficking control mechanism. A possible other role for competition between COPI and 14-3-3 binding is discussed.

The muscle-specific receptor tyrosine kinase (MuSK) is part of a receptor complex, activated by neural agrin, that orchestrates the differentiation of the neuromuscular junction (NMJ). To gain insight into the function of the MuSK complex, we have developed a proteomic approach to identify new MuSK partners. MS analysis of MuSK crosslink products from postsynaptic membranes of the Torpedo electrocytes identified the adaptor protein 14-3-3 γ. The 14-3-3 γ protein was found localized at the adult rat NMJ. Cotransfection experiments in COS-7 cells showed that MuSK codistributed with the 14-3-3 γ protein at the plasma membrane. Furthermore, 14-3-3 γ was copurified by affinity chromatography with MuSK from transfected COS-7 cells and myotubes. The 14-3-3 γ protein did not colocalize with agrin-elicited acetylcholine receptor (AChR) aggregates in cultured myotubes, suggesting that it is not involved in AChR clustering. Expression of 14-3-3 γ specifically repressed the transcription of several synaptic reporter genes in cultured myotubes. This repression was potentiated by MuSK expression. Moreover, the expression of 14-3-3 γ in muscle fibers in vivo caused both the repression of synaptic genes transcription and morphological perturbations of the NMJ. Our data extend the notion that, apart from its well documented role in AChR clustering, the MuSK complex might also be involved in the regulation of synaptic gene expression at the NMJ.

International audience; The 14-3-3 proteins constitute a family that is highly conserved in a wide range of organisms, including higher eukaryotes, invertebrates and plants. Variants of 14-3-3 proteins assembled in homo- and heterodimers were found to interact with diverse cellular proteins. Until recently, the biological role of 14-3-3 members was still poorly understood. However, the results of an increasing number of studies on their structure and function are converging to define 14-3-3 proteins as a novel type of adaptor that modulates interactions between components involved in signal transduction pathway and in cell cycle control.