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: Among our panel of Bicc1 mutations, the mutation in mutant D disrupted the largest number of intermolecular H bonds at the SAM-SAM interface (Fig. 4C and D). To distinguish whether this interface mediates Bicc1 dimerization or the formation of higher-order assemblies, we compared the sizes of wild-type Bicc1 and mutant D in transfected HEK293T cells by sucrose gradient fractionation. Analysis of polymeric HA-Bicc1 complexes in HEK293T cells revealed a broad size distribution, with wild-type HA-Bicc1 extending beyond the fractions marked by ribosomal protein S6 (RPS6) (59), indicating that wild-type Bicc1 congregates in molecular assemblies larger than ribosomes (Fig. 5A). A similar distribution was observed for endogenous Bicc1 in extracts of mIMCD3 cells (see Fig. S4B in the supplemental material). In contrast, HA-Bicc1 mutant D was concentrated in fractions with significantly lower molecular weights in three independent experiments. These data suggest that at least the largest Bicc1 assemblies in such cell extracts likely depend on SAM polymerization. However, despite this marked shift to lower-molecular-weight fractions, HA-Bicc1 mutD levels in the first three fractions did not increase, suggesting that Bicc1 likely fails to stably accumulate as a free monomer.
Affiliation: Ecole Polytechnique Fédérale de Lausanne (EPFL), SV ISREC, Lausanne, Switzerland.