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Adenosine Triphosphatases/*chemistry/metabolism
Bacterial Proteins/*chemistry/genetics/metabolism
Binding Sites
Escherichia coli/cytology/metabolism
Escherichia coli Proteins/*chemistry/genetics/metabolism
Fluorescence Resonance Energy Transfer
Humans
Mass Spectrometry
Membrane Transport Proteins/*chemistry/metabolism
Models, Molecular
Protein Binding
Protein Structure, Tertiary
Protein Subunits/chemistry/genetics/metabolism
Receptors, Cytoplasmic and Nuclear/*chemistry/genetics/metabolism
Receptors, Peptide/*chemistry/genetics/metabolism
Ribosomes/*chemistry/metabolism
SEC Translocation Channels
SecA Proteins
Signal Recognition Particle/*chemistry/metabolism
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 SRbeta subunit of the SRP receptor (SR), which probably senses a vacant translocon by direct interaction with the translocon. Bacteria lack the SRbeta 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 SRalpha 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 SRalpha and SRbeta subunits in one single protein.
