Predominant membrane localization is an essential feature of the bacterial signal recognition particle receptor

<p>Abstract</p> <p>Background</p> <p>The signal recognition particle (SRP) receptor plays a vital role in co-translational protein targeting, because it connects the soluble SRP-ribosome-nascent chain complex (SRP-RNCs) to the membrane bound Sec translocon. The eukaryotic SRP receptor (SR) is a heterodimeric protein complex, consisting of two unrelated GTPases. The SR<it>β </it>subunit is an integral membrane protein, which tethers the SRP-interacting SR<it>α </it>subunit permanently to the endoplasmic reticulum membrane. The prokaryotic SR lacks the SR<it>β </it>subunit and consists of only the SR<it>α </it>homologue FtsY. Strikingly, although FtsY requires membrane contact for functionality, cell fractionation studies have localized FtsY predominantly to the cytosolic fraction of <it>Escherichia coli</it>. So far, the exact function of the soluble SR in <it>E. coli </it>is unknown, but it has been suggested that, in contrast to eukaryotes, the prokaryotic SR might bind SRP-RNCs already in the cytosol and only then initiates membrane targeting.</p> <p>Results</p> <p>In the current study we have determined the contribution of soluble FtsY to co-translational targeting <it>in vitro </it>and have re-analysed the localization of FtsY <it>in vivo </it>by fluorescence microscopy. Our data show that FtsY can bind to SRP-ribosome nascent chains (RNCs) in the absence of membranes. However, these soluble FtsY-SRP-RNC complexes are not efficiently targeted to the membrane. In contrast, we observed effective targeting of SRP-RNCs to membrane-bond FtsY. These data show that soluble FtsY does not contribute significantly to cotranslational targeting in <it>E. coli</it>. In agreement with this observation, our <it>in vivo </it>analyses of FtsY localization in bacterial cells by fluorescence microscopy revealed that the vast majority of FtsY was localized to the inner membrane and that soluble FtsY constituted only a negligible species <it>in vivo</it>.</p> <p>Conclusion</p> <p>The exact function of the SRP receptor (SR) in bacteria has so far been enigmatic. Our data show that the bacterial SR is almost exclusively membrane-bound <it>in vivo</it>, indicating that the presence of a soluble SR is probably an artefact of cell fractionation. Thus, co-translational targeting in bacteria does not involve the formation of a soluble SR-signal recognition particle (SRP)-ribosome nascent chain (RNC) intermediate but requires membrane contact of FtsY for efficient SRP-RNC recruitment.</p

Renal Denervation in Patients with Resistant Hypertension-Assessment by 3T Renal Na-23-MRI: Preliminary Results

Background/Aim: Renal denervation (RDN) has been considered a promising therapy option for patients suffering from therapy-resistant hypertension. Besides, in blood-pressure regularization, the kidneys play a fundamental role in sodium (Na-23) homeostasis. This study assesses the effect of RDN on renal Na-23 concentration using Na-23 magnetic resonance imaging (MRI). Patients and Methods: Two patients with therapy-resistant hypertension underwent RDN. Na-23-MRI, H-1-MRI, including diffusion weighted imaging (DWI), as well as endothelial dysfunction assessment, were performed 1 day prior, as well as 1, 30 and 90 days after RDN. Results: The renal corticomedullary Na-23 gradient did not change after RDN for all time points. Additionally, functional imaging and retinal vessel parameters were not influenced by RDN. Results regarding blood pressure changes and arterial stiffness, as well as patients' clinical outcome, were heterogeneous. Conclusion: RDN does not seem to alter renal Na-23 concentration gradients, as measured by MRI