In vivo and in vitro systems for studying bacterial membrane biogenesis

The tenth membrane region of band 3 is initially exposed to the luminal side of the endoplasmic reticulum and then integrated into a partially folded band 3 intermediate

Band 3 is a typical polytopic membrane protein that mediates anion exchange activity [anion exchanger 1 (AE1)]. Although the topology and topogenesis of approximately 40 residues just after transmembrane (TM) 9 have been extensively studied, the topogenesis of this region [tenth region (10thR)] has been unclear. Glycosylation sites created in the 10thR were efficiently glycosylated in a cell-free transcription/translation system, whereas the glycosylation efficiencies were quite low in a cultured cell system. When TM12-14 was deleted or when cycloheximide was added to the culture medium, however, the glycosylation efficiency in the cultured cells increased to the same level as in the cell-free system, indicating that TM12 is essential for the sequestration from oligosaccharyl transferase into membrane and that cycloheximide treatment of the cells can mimic the cell-free system by reducing the rate of chain elongation. The glycosylation efficiency in cultured cells also increased with deletion of TM1-3. These results suggest that the 10thR is transiently extruded into the lumen and then inserted into the membrane. Both TM12 and the distant TM1-3 affect the membrane insertion of the 10thR. This indicates that during the folding of the protein, the 10thR is inserted into the membrane after the TM1-12 segments are properly assembled.

Accumulation of endoplasmic membranes and novel membrane-bound ribosome-signal recognition particle receptor complexes in Escherichia coli

In Escherichia coli, ribosomes must interact with translocons on the membrane for the proper integration of newly synthesized membrane proteins, cotranslationally. Previous in vivo studies indicated that unlike the E. coli signal recognition particle (SRP), the SRP receptor FtsY is required for membrane targeting of ribosomes. Accordingly, a putative SRP-independent, FtsY-mediated ribosomal targeting pathway has been suggested (Herskovits, A.A., E.S. Bochkareva, and E. Bibi. 2000. Mol. Microbiol. 38:927-939). However, the nature of the early contact of ribosomes with the membrane, and the involvement of FtsY in this interaction are unknown. Here we show that in cells depleted of the SRP protein, Ffh or the translocon component SecE, the ribosomal targeting pathway is blocked downstream and unprecedented, membrane-bound FtsY-ribosomal complexes are captured. Concurrently, under these conditions, novel, ribosome-loaded intracellular membrane structures are formed. We propose that in the absence of a functional SRP or translocon, ribosomes remain jammed at their primary membrane docking site, whereas FtsY-dependent ribosomal targeting to the membrane continues. The accumulation of FtsY-ribosome complexes induces the formation of intracellular membranes needed for their quantitative accommodation. Our results with E. coli, in conjunction with recent observations made with the yeast Saccharomyces cerevisiae, raise the possibility that the SRP receptor-mediated formation of intracellular membrane networks is governed by evolutionarily conserved principles.

The amino terminus of opsin translocates "posttranslationally" as efficiently as cotranslationally

Opsin, a member of the G-protein-coupled receptor family, is a polytopic membrane protein that does not encode a cleaved amino-terminal signal sequence. The amino terminus of opsin precedes the first known targeting information, suggesting that it translocates across the endoplasmic reticulum (ER) membrane after synthesis, uncoupled from translation. However, translocation across the mammalian ER is believed to be coupled to protein synthesis. In this study we show that opsin, within a range of nascent peptide lengths, targets and translocates equally efficiently co- and posttranslationally. Longer nascent opsin peptides have a lower efficiency of cotranslational translocation but an even lower efficiency of posttranslational translocation. We also show that SRP is required for both co- and posttranslational targeting.