Visualizing two root-level signature peptides in the enzyme RuBisCO. Two root-level signature peptides (green and blue surfaces) correspond to regions of the protein which cross each other at an angle to form the bottom of a hydrophobic pocket where the substrate analog inhibitor 2,2-carboxyarabinitol-1,5-bisphosphate (spheres) binds. Residues in the signature peptides interact with the substrate, but also with each other. The former interactions contribute to substrate specificity, while the latter contribute to stability. Structure coordinates from PDB entry 1WDD.