Preservation of the membrane barrier to ions by a SecYEG - ribosome - nascent chain complex
Sprache des Vortragstitels:
Mechanisms and Regulation of Protein Translocation
Sprache des Tagungstitel:
While engaged in protein transport, the bacterial translocon SecYEG must maintain the membrane barrier to small ions. The underlying mechanism is unknown. In posttranslational translocation the plug needs to be removed from the central channel enabling the passage of both the ions and the peptide chain (1). A voltage driven conformational change restored the barrier to ions in the presence of a stalled translocation intermediate (2). In cotranslational translocation the plug may remain in its channel sealing position (3) and the nascent chain may block the wider cross-sectional area of the constriction zone thereby blocking the passage of ions. We tested that hypothesis by docking a ribosome nascent chain complex onto the purified and reconstituted channel while measuring the current through a planar lipid bilayer in response to transmembrane voltage. We observed channel activity that was reminiscent to the one elucidated by empty ribosomes (4). However, there was a large increase in the probability of the channel to open as judged from the fact that single channel events were detected at much lower ligand concentration. Strikingly, the single channel conductance did not change to the same extent. At physiological membrane voltages the SecYEG-ribosome-nascent chain complex was non-conductive suggesting that a voltage dependent conformational change is essential for the maintenance of the barrier to protons and ions.