FIGURE 1. The SecYE channel¹³, with some of its binding partners and model substrates shown at the same scale.

The channel transports newly made proteins across cell membranes. Two cut-away views through its centre are shown at left and right, with the -helix that blocks the exit channel moved into the proposed 'open' position¹³. Hydrophobic amino-acid side chains are in green, acidic groups in red, basic groups in blue, and other atoms in grey; yellow ribbons denote -helices of SecY inside the surface. Space-filling representations of model substrates use the same colours. The channel's molecular dimensions are shown in the middle. The large ribosomal subunit synthesizes proteins; RNA is in grey and the subunit's protein backbone in yellow, with its exit channel positioned roughly as seen in ref. 10. A surface representation of an Hsp70 protein related to BIP shows the peptide-binding domain in yellow, bound peptide in red and ATP-hydrolysing domain in blue (with relative orientation roughly as in ref. 14); BIP prevents preproteins from slipping back into the channel. A surface representation of SecA from Bacillus subtilis shows the first (blue) and second (cyan) ATP/ADP-binding folds, amino-terminal (yellow) and carboxy-terminal (orange) preprotein-crosslinking domains, scaffold domain (dark green), wing domain (light green), and carboxy-terminal linker (red)¹⁵. This protein helps to push preproteins into the channel. The figure was created with DINO¹⁶. Protein Data Bank accession numbers are available from us on request.