Simplet/Fam53b is required for Wnt signal transduction by regulating ß-catenin nuclear localization.

Canonical ß-catenin-dependent Wnt signal transduction is important for several biological phenomena, such as cell fate determination, cell proliferation, stem cell maintenance and anterior-posterior axis formation. The hallmark of canonical Wnt signaling is the translocation of ß-catenin into the nucleus where it activates gene transcription. However, the mechanisms regulating ß-catenin nuclear localization are poorly understood. We show that Simplet/Fam53B (Smp) is required for Wnt signaling by positively regulating ß-catenin nuclear localization. In the zebrafish embryo, the loss of smp blocks the activity of two ß-catenin-dependent reporters and the expression of Wnt target genes, and prevents nuclear accumulation of ß-catenin. Conversely, overexpression of smp increases ß-catenin nuclear localization and transcriptional activity in vitro and in vivo. Expression of mutant Smp proteins lacking either the nuclear localization signal or the ß-catenin interaction domain reveal that the translocation of Smp into the nucleus is essential for ß-catenin nuclear localization and Wnt signaling in vivo. We also provide evidence that mammalian Smp is involved in regulating ß-catenin nuclear localization: the protein colocalizes with ß-catenin-dependent gene expression in mouse intestinal crypts; siRNA knockdown of Smp reduces ß-catenin nuclear localization and transcriptional activity; human SMP mediates ß-catenin transcriptional activity in a dose-dependent manner; and the human SMP protein interacts with human ß-catenin primarily in the nucleus. Thus, our findings identify the evolutionary conserved SMP protein as a regulator of ß-catenin-dependent Wnt signal transduction.

Two conserved regulatory cytoplasmic poly(A) polymerases, GLD-4 and GLD-2, regulate meiotic progression in C. elegans.

Translational regulation is heavily employed during developmental processes to control the timely accumulation of proteins independently of gene transcription. In particular, mRNA poly(A) tail metabolism in the cytoplasm is a key determinant for balancing an mRNA's translational output and its decay rate. Noncanonical poly(A) polymerases (PAPs), such as germline development defective-2 (GLD-2), can mediate poly(A) tail extension. Little is known about the regulation and functional complexity of cytoplasmic PAPs. Here we report the discovery of Caenorhabditis elegans GLD-4, a cytoplasmic PAP present in P granules that is orthologous to Trf4/5p from budding yeast. GLD-4 enzymatic activity is enhanced by its interaction with GLS-1, a protein associated with the RNA-binding protein GLD-3. GLD-4 is predominantly expressed in germ cells, and its activity is essential for early meiotic progression of male and female gametes in the absence of GLD-2. For commitment into female meiosis, both PAPs converge on at least one common target mRNA-i.e., gld-1 mRNA-and, as a consequence, counteract the repressive action of two PUF proteins and the putative deadenylase CCR-4. Together our findings suggest that two different cytoplasmic PAPs stabilize and translationally activate several meiotic mRNAs to provide a strong fail-safe mechanism for early meiotic progression.