The signal recognition particle contacts uL23 and scans substrate translation inside the ribosomal tunnel.

The signal recognition particle (SRP) delivers ∼25% of all bacterial proteins to the membrane for cotranslational insertion. However, a comprehensive model on how the low-abundant SRP scans the vast number of translating ribosomes to identify the correct substrates is lacking. Here, we show that the C-terminal helix of the signal-sequence-binding domain of SRP penetrates into the ribosomal tunnel and contacts the intra-tunnel loop of ribosomal protein uL23. This allows SRP to obtain information about the translational status of the ribosome and possibly the character of the approaching nascent chain. Correct substrates reposition the C-terminal helix of SRP, which facilitates stable binding of the signal sequence by the M-domain of SRP. Thus, SRP already surveys translating ribosomes before the signal sequence is surface exposed. This early interaction probably enables the small number of SRP molecules to scan many ribosomes and to initiate efficient targeting of proper substrates.