Browsing by Subject "Liposomen"

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Membraninsertion des Phagenproteins M13 procoat in Lipidvesikel mit rekonstituiertem Escherichia coli YidC
(2011) Stiegler, Natalie; Kuhn, Andreas
Translocation of proteins across or into the cytoplasmic membrane of Escherichia coli is accomplished by several mechanisms. The cellular secretion machinery, the translocase SecYEG, mediates the transport of unfolded proteins into the periplasm with the help of the ATPase SecA or passes the membrane proteins for bilayer integration to the insertase YidC. Membrane insertion is catalysed by YidC, whereby the native conformation of the proteins in the lipid bilayer is achieved. The translocation of a few membrane proteins occurs Sec-independently solely with the help of the insertase YidC. One of these Sec-independent proteins is the major capsid protein of the bacteriophage M13. This protein is inserted as preprotein, termed M13 procoat, with the orientation Nin-Cin into the inner membrane and a central loop domain located in the periplasm. This process is catalysed by the electrochemical membrane potential and YidC. M13 procoat is then processed by the leader peptidase to its mature form, M13 coat (orientation Nout-Cin). In the present thesis an analysis of the different transport systems of the inner membrane is performed using the example of the M13 procoat protein and its mutants. One mutant is the procoat H5EE which has 2 additional acidic residues introduced between residues +2 and +3. The insertion of this mutant requires the Sec translocase and strictly depends on the electrochemical potential. Membrane insertion of M13 procoat and derived proteins into the cytoplasmic membrane was followed in an in vitro reconstitution and translocation system. Therefore, all components of the Sec translocase (SecYEG and SecA), the insertase YidC and the different procoat proteins were purified and tested with the in vitro translocation system. Reconstitution of YidC into phospholipid vesicles depended on the lipid composition for its orientation. The cytoplasmic-out orientation corresponds to the active topology in E. coli where both termini are located in the cytoplasm. Certain lipid compositions caused the inversed orientation, which affected the catalytic activity of the reconstituted insertase. The procoat mutants H5 und H5EE were membrane inserted only in the presence of reconstituted YidC. Both proteins inserted efficiently into the vesicles with the periplasmic loop in the interior of the vesicles like the mutant PClep of procoat H5 with the C-terminal extension of the leader peptidase. Spontaneous insertion of H5 und H5EE into liposomes occurred only into leaky vesicles of the E. coli lipids. The membrane integrity was improved by the addition of an adequate amount of diacylglycerol (DAG) to the phospholipids. The leaky phospholipids were sealed by the addition of 3-4% DAG. The proteins H5 und H5EE showed a dependency of the membrane potential. Insertion occured more efficiently into YidC proteoliposomes when a stable membrane potential was generated. Proteoliposomes with reconstituted SecYEG translocase were also tested for protein insertion. Remarkedly, the protein M13 procoat H5EE efficiently inserted into SecYEG proteoliposomes, where the wildtype-like protein H5 did not.
Molekulare Dynamik der YidC-Membraninsertase aus Escherichia coli
(2011) Imhof, Nora; Kuhn, Andreas
The membrane insertase YidC of the Gram-negative bacterium E. coli enables the insertion of proteins into the cytoplasmic membrane. YidC itself is localized in the cytoplasmic membrane and spans the membrane six times with its N- and C-termini localized in the cytoplasm. These six transmembrane segments are connected by three periplasmic loops (P1, P2 and P3) and two cytoplasmic loops (C1 and C2). It is known that the binding of the YidC-dependent protein Pf3 coat induces conformational changes in the tertiary structure of YidC. This molecular dynamic of YidC was examined in detail with steady-state and time-resolved fluorescence spectroscopy. Therefore, three tryptophan mutants of YidC with one tryptophan residue each, at position 354 in the first periplasmic domain P1, at position 454 in the second periplasmic region and at position 508 near the third periplasmic region, respectively, were used. Additionally, a double tryptophan mutant was used which contained two tryptophan residues at position 332/334 of the domain P1. These tryptophan residues were used as intrinsic fluorophores. First, it was shown that the tryptophan mutants of YidC complemented the growth defect of the E. coli YidC-depletion strain JS7131. Additionally, the mutants were able to insert the strictly YidC-dependent PClep protein into the cytoplasmic membrane of the depletion strain. Thus, the functionality of the tryptophan mutants of YidC was ensured. Purified tryptophan mutants of YidC were reconstituted into liposomes and titrated with Pf3W0 coat, a tryptophan free mutant of Pf3 coat protein allowing spectroscopic studies of each periplasmic region (P1, P2 and P3) before and after binding of Pf3W0 coat protein. Analysis of the emission spectra and the fluorescence lifetimes of detergent solubilized as well as of the reconstituted YidC tryptophan mutants before binding of Pf3W0 coat revealed that the tryptophan residue of each single tryptophan mutant (YidCW354, YidCW454 and YidCW508) was localized at the membrane/water interface. These results are consistent with the proposed membrane topology of YidC. The tryptophan residues of the double tryptophan mutant of YidC (YidC2W) showed fluorescence properties consistent with their localization in a partially exposed alpha-helical segment of the P1 domain. Analysis of the emission spectra and the fluorescence lifetimes provided additional evidence that binding of Pf3W0 coat induced conformational changes of all periplasmic regions (P1, P2 and P3) within YidC. Measurements of fluorescence anisotropy showed that the conformational changes affected motions within all three periplasmic regions of the YidC tryptophan mutants, whereas the periplasmic domain P1 with the tryptophan residues W332/W334 and the third periplasmic domain P3 with the tryptophan residue W508 were affected most significantly.