RLIP,an effector of the Ral GTPases,is a platform for Cdk1 to phosphorylate epsin during the switch off of endocytosis in mitosis

The Ral signaling pathway is critically involved in Ras-dependent oncogenesis. One of its key actors, RLIP/RalBP1, which participates in receptor endocytosis during interphase, is also involved in mitotic processes when endocytosis is switched off. During mitosis, RLIP76 is located on the duplicated centrosomes and is required for their proper separation and movement to the poles. We have looked for actors that associate with RLIP during mitosis. We show here that RLIP/RalBP1 interacts with an active p34cdc2.cyclinB1 (cdk1) enzyme and that this interaction is crucial for the mitotic phosphorylation of Epsin that, once phosphorylated, is no longer competent for endocytosis. We show also that this latter phosphorylation is dependent on Ral signaling. We propose that RLIP/RalBP1 is used as a platform by the mitotic cdk1 to facilitate the phosphorylation of Epsin, which makes Epsin incompetent for endocytosis during mitosis, when endocytosis is switched off.     

Identification of the autoantigen HB as the barrier-to-autointegration factor

The HB autoantigen, a 10-kDa DNA-binding protein recognized by autoantibodies only when bound to DNA, was identified by two-dimensional electrophoresis. Silver-stained protein spots corresponding to the antigen were excised from two-dimensional electrophoresis gels, digested with trypsin, and analyzed by matrix-assisted laser desorption/ionization-reflectron time of flight and nano-electrospray ionization-ion trap/mass spectrometry. Data base search identified the HB antigen as the barrier-to-autointegration factor, a cellular protein implicated in the cellular cycle that blocks autointegration and promotes intermolecular integration of retrovirus such as the Moloney murine leukemia and the human immunodeficiency type 1 virus. The physicochemical characteristics described for these proteins, their ability to bind double-stranded DNA but not single-stranded DNA, and their nuclear localization confirm that HB and barrier-to-autointegration factor are the same protein.     

A novel germ line-specific gene of the phosducin-like protein (PhLP) family. A meiotic function conserved from yeast to mice

We identified a new member of the phosducin-like (PhLP) protein family that is predominantly, if not exclusively, expressed in male and female germ cells. In situ analysis on testis sections and analysis of purified spermatogenic cell fractions evidenced a stage-specific expression with high levels of RNA and protein in pachytene spermatocytes and round spermatids. Three mRNA species were detected, which correspond to different polyadenylation sites and vary in abundance during germ cell maturation. Only low levels of RNA were detected in whole ovary extracts, but expression of the protein became detectable within hours after hormonal induction of superovulation. The gene (Mgcphlp) is located on mouse chromosome 5 in the immediate vicinity of the Clock locus. The predicted amino acid sequence shows extensive similarities not only with the known mammalian PhLP proteins but also with the yeast phosducin-like protein Plp2, required for the production and growth of haploid cells. Expression of the murine protein was found to complement the defect of a yeast plp2 Delta mutant. We propose that MgcPhLP/Plp2 proteins exert a function in germ cell maturation that is conserved from yeast to mammals.     

Natriuretic peptide receptor A activation stabilizes a membrane-distal dimer interface

We have shown previously (Rondeau, J.-J., McNicoll, N., Gagnon, J., Bouchard, N., Ong, H., and De Léan, A. (1995) Biochemistry 34, 2130-2136) that atrial natriuretic peptide (ANP) stabilizes a dimeric form of the natriuretic peptide receptor A (NPRA) by simultaneously interacting with both receptor subunits. However, the first crystallographic study of unliganded NPRA extracellular domain documented a V-shaped dimer involving a membrane-proximal dimer interface and separate binding sites for ANP on each monomer. We explored the possibility of an alternative A-shaped dimer involving a membrane-distal dimer interface by substituting an unpaired solvent-exposed cysteine for Trp(74) in the amino-terminal lobe of full-length NPRA. The predicted spacing between Trp(74) from both subunits drastically differs, depending on whether the V-shaped (84 A) or the A-shaped (8 A) dimer model is considered. In contrast with the expected results for the reported V-shaped dimer, the NPRA(W74C) mutant was constitutively covalently dimeric. Also, the subunits spontaneously reassociated following transient disulfide reduction by dithiothreitol and reoxidation. However, ANP could neither bind to nor activate NPRA(W74C). Permanent disulfide opening by reduction with dithiothreitol and alkylation with N-ethylmaleimide rescued ANP binding to NPRA(W74C). The NPRA mutant could be maintained as a covalent dimer while preserving its function by crosslinking with the bifunctional alkylating agent phenylenedimaleimides (PDM), the ortho-substituted oPDM being more efficient than mPDM or pPDM. These results indicate that the membrane-distal lobe of the NPRAM extracellular domains are dynamically interfacing in the unliganded state and that ANP binding stabilizes the receptor dimer with more stringent spacing at the dimer interface.     

In vitro synthesis of lactose permease to probe the mechanism of membrane insertion and folding

Insertion and folding of polytopic membrane proteins is an important unsolved biological problem. To study this issue, lactose permease, a membrane transport protein from Escherichia coli, is transcribed, translated, and inserted into inside-out membrane vesicles in vitro. The protein is in a native conformation as judged by sensitivity to protease, binding of a monoclonal antibody directed against a conformational epitope, and importantly, by functional assays. By exploiting this system it is possible to express the N-terminal six helices of the permease (N(6)) and probe changes in conformation during insertion into the membrane. Specifically, when N(6) remains attached to the ribosome it is readily extracted from the membrane with urea, whereas after release from the ribosome or translation of additional helices, those polypeptides are not urea extractable. Furthermore, the accessibility of an engineered Factor Xa site to Xa protease is reduced significantly when N(6) is released from the ribosome or more helices are translated. Finally, spontaneous disulfide formation between Cys residues at positions 126 (Helix IV) and 144 (Helix V) is observed when N(6) is released from the ribosome and inserted into the membrane. Moreover, in contrast to full-length permease, N(6) is degraded by FtsH protease in vivo, and N(6) with a single Cys residue at position 148 does not react with N-ethylmaleimide. Taken together, the findings indicate that N(6) remains in a hydrophilic environment until it is released from the ribosome or additional helices are translated and continues to fold into a quasi-native conformation after insertion into the bilayer. Furthermore, there is synergism between N(6) and the C-terminal half of permease during assembly, as opposed to assembly of the two halves as independent domains.     

20 S proteasome from Saccharomyces cerevisiae is responsive to redox modifications and is S-glutathionylated

The 20 S proteasome core purified from Saccharomyces cerevisiae is inhibited by reduced glutathione (GSH), cysteine (Cys), or the GSH precursor gamma-glutamylcysteine. Chymotrypsin-like activity was more affected by GSH than trypsin-like activity, whereas the peptidylglutamyl-hydrolyzing activity (caspase-like) was not inhibited by GSH. Cys-sulfenic acid formation in the 20 S core was demonstrated by spectral characterization of the Cys-S(O)-4-nitrobenzo-2-oxa-1,3-diazole adduct, indicating that 20 S proteasome Cys residues might react with reduced sulfhydryls (GSH, Cys, and gamma-glutamylcysteine) through the oxidized Cys-sulfenic acid form. S-Glutahionylation of the 20 S core was demonstrated in vitro by GSH-biotin incorporation and by decreased alkylation with monobromobimane. Compounds such as N-ethylmaleimide (-S-sulfhydril H alkylating), dimedone (-SO sulfenic acid H reactant), or 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (either -SH or -SOH reactant) highly inhibited proteasomal chymotrypsin-like activity. In vivo experiments revealed that 20 S proteasome extracted from H(2)O(2)-treated cells showed decreased chymotrypsin-like activity accompanied by S-glutathionylation as demonstrated by GSH release from the 20 S core after reduction with NaBH(4). Moreover, cells pretreated with H(2)O(2) showed decreased reductive capacity assessed by determination of the GSH/oxidized glutathione ratio and increased protein carbonyl levels. The present results indicate that at the physiological level the yeast 20 S proteasome is regulated by its sulfhydryl content, thereby coupling intracellular redox signaling to proteasome-mediated proteolysis.     

Solution structure of the recombinant penaeidin-3, a shrimp antimicrobial peptide

Penaeidins are a family of antimicrobial peptides of 47-63 residues isolated from several species of shrimp. These peptides display a proline-rich domain (N-terminal part) and a cysteine-rich domain (C-terminal part) stabilized by three conserved disulfide bonds whose arrangement has not yet been characterized. The recombinant penaeidin-3a of Litopenaeus vannamei (63 residues) and its [T8A]-Pen-3a analogue were produced in Saccharomyces cerevisiae and showed similar antimicrobial activity. The solution structure of the [T8A]-Pen-3a analogue was determined by using two-dimensional 1H NMR and simulated annealing calculations. The proline-rich domain, spanning residues 1-28 was found to be unconstrained. In contrast, the cysteine-rich domain, spanning residues 29-58, displays a well defined structure, which consists of an amphipathic helix (41-50) linked to the upstream and the downstream coils by two disulfide bonds (Cys32-Cys47 and Cys48-Cys55). These two coils are in turn linked together by the third disulfide bond (Cys36-Cys54). Such a disulfide bond packing, which is in agreement with the analysis of trypsin digests by ESI-MS, contributes to the highly hydrophobic core. Side chains of Arg45 and Arg50, which belong to the helix, and side chains of Arg37 and Arg53, which belong to the upstream and the downstream coils, are located in two opposite parts of this globular and compact structure. The environment of these positively charged residues, either by hydrophobic clusters at the surface of the cysteine-rich domain or by sequential hydrophobic residues in the unconstrained proline-rich domain, gives rise to the amphipathic character required for antimicrobial peptides. We hypothesize that the antimicrobial activity of penaeidins can be explained by a cooperative effect between the proline-rich and cysteine-rich features simultaneously present in their sequences.     

Phosphatidylinositol 3-kinase interacts with the adaptor protein Dab1 in response to Reelin signaling and is required for normal cortical lamination

Reelin is a large secreted signaling protein that binds to two members of the low density lipoprotein receptor family, the apolipoprotein E receptor 2 and the very low density lipoprotein receptor, and regulates neuronal positioning during brain development. Reelin signaling requires activation of Src family kinases as well as tyrosine phosphorylation of the intracellular adaptor protein Disabled-1 (Dab1). This results in activation of phosphatidylinositol 3-kinase (PI3K), the serine/threonine kinase Akt, and the inhibition of glycogen synthase kinase 3beta, a protein that is implicated in the regulation of axonal transport. Here we demonstrate that PI3K activation by Reelin requires Src family kinase activity and depends on the Reelin-triggered interaction of Dab1 with the PI3K regulatory subunit p85alpha. Because the Dab1 phosphotyrosine binding domain can interact simultaneously with membrane lipids and with the intracellular domains of apolipoprotein E receptor 2 and very low density lipoprotein receptor, Dab1 is preferentially recruited to the neuronal plasma membrane, where it is phosphorylated. Efficient Dab1 phosphorylation and activation of the Reelin signaling cascade is impaired by cholesterol depletion of the plasma membrane. Using a neuronal migration assay, we also show that PI3K signaling is required for the formation of a normal cortical plate, a step that is dependent upon Reelin signaling.