A crystallographic hexamer can be produced when a two-fold rotational symmetry operation is applied to the three molecules (one and a half dimers) in the asymmetric unit ( Figure 5 ). In this hexamer, the N-lobes of three kinase molecules are pointing to the same side of the ''hexamerization plane'', while the N-lobes of the other three molecules are pointing to the opposite side of the plane (Figure 5b ). This assembly mode of the kinase domain allows locating the epimerase domain further away from the hexamerization plane and is consistent with the proposition that the interdomain segment (residues 360-382) is the site of trimerization [18] .