Comparisons between the computationally minimized structures for the complex of EpoB bound to Tub2-BBBBB tubulin (grey structures in all panels) vs. EpoB in complex with the Tub2-YBBBB (left-hand quadrants) or Tub2-BBBYB (right quadrants) tubulin mutants (shown as green-colored sticks in all panels). The two top quadrants depict complexes solved based on ligand-binding according to NMR characterization (12), while the two bottom quadrants depict complexes derived from assuming binding modes analogous to the EC prediction (9). All heteroatoms within the ligand (EpoB) and key receptor residues (tubulin) are colored according to standard CPK coloring; tubulin secondary structure is rendered as tan ribbons. Thick black arrows denote favorable interactions enjoyed by the Tub2-BBBBB complex but absent for corresponding mutants. Thin dashed arrows are used to indicate the position of residue 227.