FIGURE 2: SecA-dependent co-translational targeting of secretory proteins to the SecYEG translocon. (A) Scanning of the ribosomal tunnel by SecA. I. SecA contacts the intra-tunnel loop of uL23 II. In the presence of an emerging polypeptide, SecA retracts from the tunnel interior. III. SecA traps the signal sequence. N and C correspond to the N-terminus and C-terminus of SecA, respectively. (B) Co-translational targeting by SecA. I. SecA targets the nascent secretory protein to the SecYEG channel. At the membrane, SecA is released from the ribosome and binds to the SecYEG translocon. Whether the secretory protein is completely released from the ribosome before translocation starts (IIa) or whether translocation starts already during ongoing translation is unknown (IIb).