Fig. 4. (A) Surface representation of M-cyclin (gold, left) and cyclin A (purple, right) bound to their respective cdk2 partners (cyan C_α worm). The cdk2 PSTAIRE region is shown in red and the T-loop in yellow for both complexes as in Figure 2. The green surface of M-cyclin and cyclin A indicates the hydrophobic patch, which packs against the PSTAIRE helix. The shift in position of M-cyclin relative to cyclin A can be seen as well as the different contacts to the PSTAIRE helix. (B) Close-up of the hydrophobic patch of M-cyclin–cdk2 and cyclin A–cdk2 revealing differences in their interface contacts. (C) Stereo view of a conserved cyclin–cdk2 interaction centred on the salt bridge between K104^M and E133^M (top view) equivalent to K266^A and E295^A (bottom view) as described in the text. Certain residues in this region are omitted for clarity.