Bicc1 Polymerization Regulates the Localization and Silencing of Bound mRNA.
Bottom Line: In addition, defective polymerization decreases Bicc1 stability and thus indirectly attenuates inhibition of Dishevelled 2 in the Wnt/β-catenin pathway.Importantly, aberrant C-terminal extension of the SAM domain in bpk mutant Bicc1 phenocopied these defects.We conclude that polymerization is a novel disease-relevant mechanism both to stabilize Bicc1 and to present associated mRNAs in specific silencing platforms.
Affiliation: Ecole Polytechnique Fédérale de Lausanne (EPFL), SV ISREC, Lausanne, Switzerland.Show MeSH
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Mentions: Bicc1 is localized in cytoplasmic foci by its SAM domain independently of the RNA-binding KH domains (35). Certain SAM domains can form dimers or polymeric structures (57), and a polymer of the human Bicc1 SAM domain fused to GFP has been observed in vitro by electron microscopy (EM) (42). To test whether SAM polymerization is responsible for Bicc1 clustering, we searched for mutations that specifically disrupt polymerization. Since structure data for Bicc1 or its SAM domain are currently unavailable, mutations were designed on the basis of homology modeling, where the known structure of a related protein serves as a template. Among the available templates, we selected the SAM domain dimer of the diacylglycerol kinase δ1 (DGKδ1) E35G (PDB accession number 3BQ7) because it shares the highest sequence similarity (54%) and identity (31%) with the Bicc1 SAM domain (Fig. 3A) and can form head-to-tail polymers (47). A model of dimeric Bicc1 SAM obtained after energy minimization revealed a common globular fold of five α helices, with two SAM subunits being docked to one another at characteristic ML and EH surfaces (Fig. 3B) (40). Residues involved in the dimerization of the DGKδ1 SAM are conserved or replaced by similar amino acids in the Bicc1 SAM domain (highlighted in Fig. 3A). At the Bicc1 SAM-SAM interface, 4 negatively charged amino acids on the ML surface (Glu900, Asp902, Asp913, Glu916) and 5 positively charged amino acids from the EH surface (Lys891, Lys915, Arg925, Arg926, Lys927) form strongly polarized electrostatic networks in two independent regions of contact (Fig. 3C and D; see also Fig. S5 in the supplemental material). In addition, residue Phe922 from the EH surface reaches into a hydrophobic pocket of the ML surface comprising Phe896, Ile901, Leu909, and Leu917 (Fig. 3E).
Affiliation: Ecole Polytechnique Fédérale de Lausanne (EPFL), SV ISREC, Lausanne, Switzerland.