The tandem PDZ domains of syntenin promote cell invasion

Syntenin is a tandem PDZ protein that has recently been shown to be overexpressed in several cancer cells and tissues, and that might play an active role in tumor cell invasion and metastasis. Here we show that overexpression of the tandem PDZ domains of syntenin in non-invasive cells is necessary and sufficient to stimulate these cells to invade a collagen I matrix, and this effect can be regulated by ligand binding to the PDZ domains. Furthermore, we show that syntenin-induced invasion requires signaling through ras, rho and PI3K/MAPK signaling pathways and involves changes in cell-cell adhesion. Inversely, when we used RNA interference to inhibit syntenin expression in different invasive cancer cell lines, we observed a drastically decreased ability of these cells to migrate and invade into collagen type I or Matrigel. RNAi-treated cells also show increased cell aggregation, indicating that syntenin is important for cell-cell adhesion in epithelial cells. Together, these results suggest that downregulation of syntenin by RNA interference could provide a means of inhibiting tumor invasion and possibly metastasis in different cancers, and point to syntenin as a potential cancer biomarker and drug target.     

Regulation of the epithelial calcium channel TRPV6 by the serum and glucocorticoid-inducible kinase isoforms SGK1 and SGK3

Epithelial calcium (re)absorption is mediated by TRPV5 and TRPV6 channels. TRPV5 is modulated by the SGK1 kinase, a process requiring the PDZ-domain containing scaffold protein NHERF2. The present study explored whether TRPV6 is similarly regulated by SGKs and the scaffold proteins NHERF1/2. In Xenopus oocytes, SGKs activate TRPV6 by increasing its plasma membrane abundance. Deletion of the putative PDZ binding motif on TRPV6 did not abolish channel activation by SGKs. Furthermore, coexpression of neither NHERF1 nor NHERF2 affected TRPV6 or potentiated the SGKs stimulating effect. The present observations disclose a novel TRPV6 regulatory mechanism which presumably participates in calcium homeostasis.     

CSF-1 and TPA stimulate independent pathways leading to lysosomal degradation or regulated intramembrane proteolysis of the CSF-1 receptor

The CSF-1 receptor is a protein-tyrosine kinase that has been shown to undergo regulated intramembrane proteolysis, or RIPping. Here, we have compared receptor downregulation and RIPping in response to CSF-1 and TPA. Our studies show that CSF-1 is a relatively poor inducer of RIPping and that CSF-1-induced receptor downregulation is largely independent of RIPping. TPA is a strong inducer of RIPping and TPA-induced receptor downregulation is mediated by RIPping. We further found that RIPping is dependent on TACE or a TACE-like protease, that CSF-1 and TPA use independent pathways to initiate RIPping, and that the intracellular domain is targeted for degradation through ubiquitination.     

Signal sequence recognition and protein targeting

Intracellular traffic is often controlled not by highways, but by handshakes and partner introductions within a cellular network. Recently determined structures suggest how signal sequences are recognized and how the GTP affinities of the signal recognition particle and its receptor are coupled to the targeting of ribosomes to translocational membrane pores. The structure of signal peptidase suggests how it releases functional proteins.     

The structure, organization, activation and plasticity of the erythropoietin receptor

Dimerization of the erythropoietin receptor has long been accepted as the singular step in its mechanism of activation. Recent studies have revealed a regulator process for activation that is dependent on the actual configuration of the receptor-ligand dimer assembly. This aspect of the receptor subunit assembly appears to extend to the unliganded receptor, which can dimerize on the cell surface and diminish any spontaneous background signaling in the absence of ligand. This self-recognition, as well as the multiple ligand binding capabilities of the receptor binding site, is consistent with an emerging theme of plasticity in protein-protein and ligand-receptor interactions.     

Structural and biochemical implications of single amino acid substitutions in the nucleotide-dependent switch regions of the nitrogenase Fe protein from<Emphasis Type="Italic"> Azotobacter vinelandii</Emphasis>

The structures of nitrogenase Fe proteins with defined amino acid substitutions in the previously implicated nucleotide-dependent signal transduction pathways termed switch I and switch II have been determined by X-ray diffraction methods. In the Fe protein of nitrogenase the nucleotide-dependent switch regions are responsible for communication between the sites responsible for nucleotide binding and hydrolysis and the [4Fe-4S] cluster of the Fe protein and the docking interface that interacts with the MoFe protein upon macromolecular complex formation. In this study the structural characterization of the Azotobacter vinelandii nitrogenase Fe protein with Asp at position 39 substituted by Asn in MgADP-bound and nucleotide-free states provides an explanation for the experimental observation that the altered Fe proteins form a trapped complex subsequent to a single electron transfer event. The structures reveal that the substitution allows the formation of a hydrogen bond between the switch I Asn39 and the switch II Asp125. In the structure of the native enzyme the analogous interaction between the side chains of Asp39 and Asp125 is precluded due to electrostatic repulsion. These results suggest that the electrostatic repulsion between Asp39 and Asp125 is important for dissociation of the Fe protein:MoFe protein complex during catalysis. In a separate study, the structural characterization of the Fe protein with Asp129 substituted by Glu provides the structural basis for the observation that the Glu129-substituted variant in the absence of bound nucleotides has biochemical properties in common with the native Fe protein with bound MgADP. Interactions of the longer Glu side chain with the phosphate binding loop (P-loop) results in a similar conformation of the switch II region as the conformation that results from the binding of the phosphate of ADP to the P-loop.