title: Signal sequence-independent SRP-SR complex formation at the membrane suggests an alternative targeting pathway within the SRP cycle.
abstract: Protein targeting by the signal recognition particle (SRP) and the bacterial SRP receptor FtsY requires a series of closely coordinated steps that monitor the presence of a substrate, the membrane, and a vacant translocon. Although the influence of substrate binding on FtsY-SRP complex formation is well documented, the contribution of the membrane is largely unknown. In the current study, we found that negatively charged phospholipids stimulate FtsY-SRP complex formation. Phospholipids act on a conserved positively charged amphipathic helix in FtsY and induce a conformational change that strongly enhances the FtsY-lipid interaction. This membrane-bound, signal sequence-independent FtsY-SRP complex is able to recruit RNCs to the membrane and to transfer them to the Sec translocon. Significantly, the same results were also observed with an artificial FtsY-SRP fusion protein, which was tethered to the membrane via a transmembrane domain. This indicates that substrate recognition by a soluble SRP is not essential for cotranslational targeting in Escherichia coli. Our findings reveal a remarkable flexibility of SRP-dependent protein targeting, as they indicate that substrate recognition can occur either in the cytosol via ribosome-bound SRP or at the membrane via a preassembled FtsY-SRP complex.
authors.affiliation: Institut für Biochemie und Molekularbiologie, ZBMZ, 79104 Freiburg, Germany.
authors.forename: David
authors.initials: D
authors.lastname: Braig
authors.forename: Miryana
authors.initials: M
authors.lastname: Mircheva
authors.forename: Ilie
authors.initials: I
authors.lastname: Sachelaru
authors.forename: Eli O
authors.initials: EO
authors.lastname: van der Sluis
authors.forename: Lukas
authors.initials: L
authors.lastname: Sturm
authors.forename: Roland
authors.initials: R
authors.lastname: Beckmann
authors.forename: Hans-Georg
authors.initials: HG
authors.lastname: Koch
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D019286:Endopeptidase K / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D008562:Membrane Glycoproteins / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism; D010743:Phospholipids / Q000378:metabolism; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D018160:Receptors, Cytoplasmic and Nuclear / Q000737:chemistry* / Q000378:metabolism*; D018000:Receptors, Peptide / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D043244:Ribonuclease III / Q000378:metabolism; D012270:Ribosomes / Q000378:metabolism; D018271:Signal Recognition Particle / Q000737:chemistry* / Q000378:metabolism*; D013329:Structure-Activity Relationship
chemical_list: D001426:Bacterial Proteins; D002135:Calcium-Binding Proteins; D029968:Escherichia coli Proteins; C087406:FtsY protein, Bacteria; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D010743:Phospholipids; D018160:Receptors, Cytoplasmic and Nuclear; D018000:Receptors, Peptide; D011993:Recombinant Fusion Proteins; D018271:Signal Recognition Particle; C066973:signal sequence receptor; D006160:Guanosine Triphosphate; D043244:Ribonuclease III; C468366:ribonuclease III, E coli; D019286:Endopeptidase K