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Cancer Networks: Solving a Tumor Suppressor Protein

SBKB [doi:10.1038/sbkb.2012.168]
Featured Article - November 2013
Short description: Structural characterization of CDK2AP1 reveals a disordered N-terminal region and a four-helix C-terminal domain.

Top-down view of the four-helix dimeric structure of CDK2AP1(61–115). The thiol groups of the two Cys105 residues (shown in yellow) are poised for disulfide formation. Reprinted with permission from ASBMB. 1

Cyclin-dependent kinase 2-associated protein 1 (CDK2AP1) is a highly conserved tumor suppressor protein encoded by the doc-1 (deleted in oral cancer 1) gene, which is absent or downregulated in many cancer cell types. Among its putative binding partners is cell cycle- regulatory kinase CDK2. CDK2AP1 is thought to inhibit CDK2's kinase activity by sequestering the inactive, nonphosphorylated form of CDK2, causing inhibition of cell cycle progression, which is consistent with previous observations and could account for its tumor suppressor function. Montelione and colleagues (PSI NESG) reported the solution structure of CDK2AP1 by NMR and hydrogen/deuterium exchange with mass spectrometry (HDX-MS) analyses.

The 60-residue N-terminal region is intrinsically disordered based on HDX-MS analysis, which led the authors to design a truncated construct corresponding to residues 61–115 for NMR studies. Previous studies suggested that CDK2AP1 forms a dimer, which was confirmed by analytical gel filtration with static light scattering, sedimentation velocity analytical ultracentrifugation and NMR relaxation measurements. The solution structure of dimeric CDK2AP1(61–115) revealed a four-helix bundle structure that is predominantly stabilized by hydrophobic interactions involving numerous residues at the dimer interface (PDB 2KW6).

It had also been previously proposed that interchain disulfide bond formation between Cys105 is essential for dimerization. However, the structural studies in this work, which were carried out under reducing conditions, do not support this idea. Indeed, a comparison of the correlation spectra of wild-type CDK2AP1 to that of a C105A mutant confirmed that the two structures are essentially identical. The authors propose that the extensive network of hydrophobic interactions is sufficient to retain dimeric structure. Nevertheless, the thiol groups of the two reduced Cys105 residues from each monomer are in relatively close proximity and are thus poised for disulfide bond formation.

Finally, Ser46 in the intrinsically disordered portion of CDK2AP1 is a predicted target of IκB kinase ε (IKKε). In vitro phosphorylation studies validate this prediction, while NMR analysis shows that there are no substantial structural changes due to this phosphorylation event. It will be interesting to investigate the functional significance of IKKε-mediated phosphorylation of CDK2AP1.

Arianne Heinrichs

References

  1. A. Ertekin et al. Human cyclin-dependent kinase 2-associated protein 1 (CDK2AP1) is dimeric in its disulphide-reduced state, with natively disordered N-terminal region.
    J. Biol. Chem. 20, 16541-16549 (2012). doi:10.1074/jbc.M112.343863

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