292 research outputs found
The Monge-Ampère equation and warped products of higher rank
No full textWe show that a warped product Mf = ℝ × f ℝ has higher rank and nonpositive curvature if and only if f is a convex solution of the Monge-Ampère equation. In this case we show that M contains a Euclidean factor. © 2007 Australian Mathematical Society
An Upper Limit on the Branching Ratio for
No full textWe have searched for decays of the ø lepton into seven or more charged particles, using data collected with the OPAL detector from 1990 to 1995 in e + e \Gamma collisions at p s ß MZ . No candidate events were found and an upper limit on the branching ratio for ø decays into seven charged particles of 1:8 \Theta 10 \Gamma5 at the 95% confidence level was determined. (Accepted by Physics Letters B) The OPAL Collaboration K. Ackerstaff 8 , G. Alexander 23 , J. Allison 16 , N. Altekamp 5 , K.J. Anderson 9 , S. Anderson 12 , S. Arcelli 2 , S. Asai 24 , D. Axen 29 , G. Azuelos 18;a , A.H. Ball 17 , E. Barberio 8 , R.J. Barlow 16 , R. Bartoldus 3 , J.R. Batley 5 , S. Baumann 3 , J. Bechtluft 14 , C. Beeston 16 , T. Behnke 8 , A.N. Bell 1 , K.W. Bell 20 , G. Bella 23 , S. Bentvelsen 8 , P. Berlich 10 , S. Bethke 14 , O. Biebel 14 , A. Biguzzi 5 , S.D. Bird 16 , V. Blobel 27 , I.J. Bloodworth 1 , J.E. Bloomer 1 , M. Bobins..
SecB—A chaperone dedicated to protein translocation
No full textSecB is a molecular chaperone in Gram-negative bacteria dedicated to the post-translational translocation of proteins across the cytoplasmic membrane. The entire surface of this chaperone is used for both of its native functions in protein targeting and unfolding. Single molecule studies revealed how SecB affects the folding pathway of proteins and how it prevents the tertiary structure formation and aggregation to support protein translocation.
F1F0 ATP synthase subunit c is a substrate of the novel YidC pathway for membrane protein biogenesis
No full textThe Escherichia coli YidC protein belongs to the Oxa1 family of membrane proteins that have been suggested to facilitate the insertion and assembly of membrane proteins either in cooperation with the Sec translocase or as a separate entity. Recently, we have shown that depletion of YidC causes a specific defect in the functional assembly of F1F0 ATP synthase and cytochrome o oxidase. We now demonstrate that the insertion of in vitro–synthesized F1F0 ATP synthase subunit c (F0c) into inner membrane vesicles requires YidC. Insertion is independent of the proton motive force, and proteoliposomes containing only YidC catalyze the membrane insertion of F0c in its native transmembrane topology whereupon it assembles into large oligomers. Co-reconstituted SecYEG has no significant effect on the insertion efficiency. Remarkably, signal recognition particle and its membrane-bound receptor FtsY are not required for the membrane insertion of F0c. In conclusion, a novel membrane protein insertion pathway in E. coli is described in which YidC plays an exclusive role.
Reshaping of the conformational search of a protein by the chaperone trigger factor
No full textProtein folding is often described as a search process, in which polypeptides explore different conformations to find their native structure. Molecular chaperones are known to improve folding yields by suppressing aggregation between polypeptides before this conformational search starts, as well as by rescuing misfolds after it ends. Although chaperones have long been speculated to also affect the conformational search itself—by reshaping the underlying folding landscape along the folding trajectory—direct experimental evidence has been scarce so far. In Escherichia coli, the general chaperone trigger factor (TF) could play such a role. TF has been shown to interact with nascent chains at the ribosome, with polypeptides released from the ribosome into the cytosol, and with fully folded proteins before their assembly into larger complexes. To investigate the effect of TF from E. coli on the conformational search of polypeptides to their native state, we investigated individual maltose binding protein (MBP) molecules using optical tweezers. Here we show that TF binds folded structures smaller than one domain, which are then stable for seconds and ultimately convert to the native state. Moreover, TF stimulates native folding in constructs of repeated MBP domains. The results indicate that TF promotes correct folding by protecting partially folded states from distant interactions that produce stable misfolded states. As TF interacts with most newly synthesized proteins in E. coli, we expect these findings to be of general importance in understanding protein folding pathways.
Tight Hydrophobic Contacts with the SecB Chaperone Prevent Folding of Substrate Proteins
No full textThe molecular chaperone SecB binds to hydrophobic sections of unfolded secretory proteins and thereby prevents their premature folding prior to secretion by the translocase of Escherichia coli. Here, we have investigated the effect of the single-residue mutation of leucine 42 to arginine (L42R) centrally positioned in the polypeptide binding pocket of SecB on its chaperonin function. The mutant retains its tetrameric structure and SecA targeting function but is defective in its holdase activity. Isothermal titration calorimetry and single-molecule optical tweezer studies suggest that the SecB(L42R) mutant exhibits a reduced polypeptide binding affinity allowing for partial folding of the bound polypeptide chain rendering it translocation-incompetent.
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