No abstract

Signal recognition particle (SRP)-dependent protein targeting is a universally conserved process that delivers proteins to the bacterial cytoplasmic membrane or to the endoplasmic reticulum membrane in eukaryotes. Crucial during targeting is the transfer of the ribosome-nascent chain complex (RNC) from SRP to the Sec translocon. In eukaryotes, this step is co-ordinated by the SRβ subunit of the SRP receptor (SR), which probably senses a vacant translocon by direct interaction with the translocon. Bacteria lack the SRβ subunit and how they co-ordinate RNC transfer is unknown. By site-directed cross-linking and fluorescence resonance energy transfer (FRET) analyses, we show that FtsY, the bacterial SRα homologue, binds to the exposed C4/C5 loops of SecY, the central component of the bacterial Sec translocon. The same loops serve also as binding sites for SecA and the ribosome. The FtsY-SecY interaction involves at least the A domain of FtsY, which attributes an important function to this so far ill-defined domain. Binding of FtsY to SecY residues, which are also used by SecA and the ribosome, probably allows FtsY to sense an available translocon and to align the incoming SRP-RNC with the protein conducting channel. Thus, the Escherichia coli FtsY encompasses the functions of both the eukaryotic SRα and SRβ subunits in one single protein.

The YidC insertase also integrates multispanning membrane proteins that had been considered to be exclusively SecYEG dependent. Only membrane proteins that require SecA can be inserted only via SecYEG. Targeting to YidC is SRP dependent, and the C-terminus of YidC cross-links to SRP, FtsY, and ribosomal subunits.

Cotranslational protein targeting delivers proteins to the bacterial cytoplasmic membrane or to the eukaryotic endoplasmic reticulum membrane. The signal recognition particle (SRP) binds to signal sequences emerging from the ribosomal tunnel and targets the ribosome-nascent-chain complex (RNC) to the SRP receptor, termed FtsY in bacteria. FtsY interacts with the fifth cytosolic loop of SecY in the SecYEG translocon, but the functional role of the interaction is unclear. By using photo-cross-linking and fluorescence resonance energy transfer measurements, we show that FtsY–SecY complex formation is guanosine triphosphate independent but requires a phospholipid environment. Binding of an SRP–RNC complex exposing a hydrophobic transmembrane segment induces a rearrangement of the SecY–FtsY complex, which allows the subsequent contact between SecY and ribosomal protein uL23. These results suggest that direct RNC transfer to the translocon is guided by the interaction between SRP and translocon-bound FtsY in a quaternary targeting complex.

No abstract

Our study reveals an alternative route in the SRP-dependent protein targeting pathway that includes a preassembled, membrane-bound SRP-SR complex. This alternative route is fully sufficient to maintain cell viability in the absence of a soluble SRP.

SummaryBackgroundTo evaluate medical versus interventional treatment (transjugular thrombus fragmentation, local thrombolysis with or without stent implantation) in patients with acute non‐cirrhotic, non‐malignant portal vein thrombosis (PVT).MethodsThis prospective, observational study enrolled 65 patients with acute (<28 days since begin of symptoms, no cavernoma) PVT in nine centres. Thirty patients received medical treatment and 35 patients received interventional treatment. PVT was graded into grade 1: short thrombosis and incomplete occlusion of the vessel lumen and grade 2: extended thrombosis or complete occlusion. Treatment response was classified as partial or complete, if thrombosis was reduced by one grade or to <25% of the vessel diameter respectively.ResultsPartial and complete response rates were 7% and 30% in the medical compared to 17% and 54% (P < 0.001) in the interventional treatment group. In the multivariate analysis, interventional treatment showed a strong positive (OR 4.32, P < 0.016) and a myeloproliferative aetiology a negative (OR 0.09, P = 0.006) prediction of complete response. Complications were rare in the medical group and consisted of septicaemia and upper gastrointestinal bleeding of unknown origin in one patient each. Interventional treatment was accompanied by mild and self‐limiting bleeding complications in nine patients, moderate intra‐abdominal bleeding requiring transfusions (2 units) in one patient and peritoneal bleeding requiring surgical rescue in one patient. Four patients in each group developed intestinal gangrene requiring surgery. One patient died 52 days after unsuccessful interventional treatment.ConclusionsCompared to medical treatment alone, interventional treatment doubled response rates at the cost of increased bleeding complications.

Background and objective: Treatment with proton pump inhibitors (PPIs) has been associated with development of hepatic encephalopathy (HE). As development of HE is a major complication after implantation of a transjugular intrahepatic portosystemic shunt (TIPS), we hypothesized that PPI treatment may be associated with a higher risk of post-TIPS HE. Methods: We analyzed data of 397 patients with liver cirrhosis who received de novo TIPS implantation at the University Medical Center Freiburg, Germany. We assessed whether PPI medication and other patient characteristics are predictive factors for the development of post-TIPS HE. Results: Patients with PPI treatment at the time of TIPS implantation showed significantly higher rates of post-TIPS HE than those without PPI medication (30.4% vs 11.7%, p < 0.001). The rate of post-TIPS HE increased in a dose-dependent manner. However, PPI medication did not directly affect transplant-free survival. Remarkably, in 59.1% of patients who received PPIs there was no clear indication. Conclusions: PPI treatment may be an independent risk factor for the development of post-TIPS HE and the risk increases with PPI dose. Indication for PPI treatment should be assessed carefully prior to TIPS implantation in patients with liver cirrhosis.