Septins constitute a group of GTP binding proteins that assemble into homo- and hetero-oligomeric complexes and filaments. These higher order septin structures are thought to function like scaffolds and/or diffusion barriers serving as spatial localizers for many proteins with key roles in cell polarity and cell cycle progression. In this study, we extensively characterized septin interaction partners using yeast two-hybrid and three-hybrid systems in addition to precipitation analyses in platelets. As a result, we identified human hetero-trimeric septin complexes on a large scale, which had been only postulated in the past. In addition, we illustrated roles of SEPT9 that might contribute to hetero-trimeric septin complex formation. SEPT9 can substitute for septins of the SEPT2 group and partially for SEPT7. Mutagenic analyses revealed that mutation of a potential phosphorylation site in SEPT7 (Y318) regulates the interaction with other septins. We identified several septin-septin interactions in platelets suggesting a regulatory role of diverse septin complexes in platelet function.

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Background: Bicaudal-C RNA binding proteins are important translational repressors in many different biological contexts. Results: A minimal site for RNA binding and translation repression by Bicaudal-C was identified. Conclusion: The RNA binding site for Bicaudal-C forms a stem-loop structure important for binding and translational repression.Significance: This will facilitate the identification of Bicaudal-C binding sites in other mRNAs.

Vertebrate Bicaudal-C (Bicc1) has important biological roles in the formation and homeostasis of multiple organs, but direct experiments to address the role of maternal Bicc1 in early vertebrate embryogenesis have not been reported. Here, we use antisense phosphorothioate-modified oligonucleotides and the host-transfer technique to eliminate specifically maternal stores of both bicc1 mRNA and Bicc1 protein from Xenopus laevis eggs. Fertilization of these Bicc1-depleted eggs produced embryos with an excess of dorsal-anterior structures and overexpressed organizer-specific genes, indicating that maternal Bicc1 is crucial for normal embryonic patterning of the vertebrate embryo. Bicc1 is an RNAbinding protein with robust translational repression function. Here, we show that the maternal mRNA encoding the cell-fate regulatory protein Wnt11b is a direct target of Bicc1-mediated repression. It is well established that the Wnt signaling pathway is crucial to vertebrate embryogenesis. Thus, the work presented here links the molecular function of Bicc1 in mRNA target-specific translation repression to its biological role in the maternally controlled stages of vertebrate embryogenesis.

Septins are a family of GTP-binding proteins, which are essential for active membrane movement such as cytokinesis and vesicle trafficking. In non-dividing cells (such as platelets and neurons) septins are implicated in exocytosis. Platelets from a SEPT5 knockout mouse showed an altered serotonin secretion and platelet aggregation, suggesting that SEPT5 is involved in secretion in platelets. Septins form complexes consisting of multiple septin polypeptides. Using the yeast two-hybrid system we had demonstrated that SEPT5 partners with SEPT8. The aim of this study was to identify other interaction partners of the human platelet septin SEPT8. Using the yeast two-hybrid system with SEPT8 as bait protein we identified the human septin SEPT4 as an interaction partner of SEPT8. The interaction between SEPT4 and SEPT8 was confirmed by immunoprecipitation. Expression analysis revealed that SEPT4 is also expressed in human platelets. Thus, SEPT4 is the third described platelet septin besides SEPT5 and SEPT8. Transmission electron microscopy of platelets revealed that SEPT8 and SEPT4 are localized surrounding alpha-granules (as it had been shown for the septin SEPT5) suggesting that the three septins may be components of the septin complex in platelets and contribute in such a way to platelet biology. Activation of platelets by agonists resulted in the translocation of SEPT4 and SEPT8 to the platelet surface indicating a possible functional role of these proteins in platelet granular secretion.