NMR structural characterization of the CDK inhibitor p19INK4d.

p19INK4d is a 165 amino acid protein that belongs to the INK4 family of CDK4 and CDK6 inhibitors. Assignments of 1H, 15N and 13C resonances have enabled the determination of the secondary structure of the protein which is largely alpha-helical (residues 14-18, 21-29, 54-62, 77-83, 87-95, 110-116, 120-128, 142-148 and 152-160). The protein comprises five 32-amino acid ankyrin-like repeats; each ankyrin repeat contains a helix-beta-turn-helix core. The exception is the second ankyrin repeat, which lacks the first helix. All beta-turns have a central glycine residue flanked by two residues in beta-conformations. There is also a high conservation of Ala at position 8 in the first helix and Leu-Leu(Val) at positions 17-18 of the second helix in all ankyrin repeats of p19. The location of the helix-turn-helix segments found in p19 should be general for all other members of the INK4 family, including, for example, a homologous tumor suppressor p16INK4a. 1H-15N heteronuclear steady-state NOE measurements on p19 indicate that most of the backbone of p19INK4d exists in a well defined structure of limited conformational flexibility on the nano- to picosecond time scale.