Dynamic equilibrium engagement of a polyvalent ligand with a single-site receptor

doi:10.1073/pnas.0809222105

Dynamic equilibrium engagement of a polyvalent ligand with a single-site receptor

^aProgram in Molecular Structure and Function, Hospital for Sick Children, 555 University Avenue, Toronto, ON, Canada M5G 1X8;
^bCentre for Systems Biology, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue, Toronto, ON, Canada M5G 1X5; and
Departments of ^cBiochemistry,
^dMolecular Genetics, and
^eChemistry, University of Toronto, 1 King's College Circle, Toronto, ON, Canada M5S 1A8

Communicated by Ada Yonath, Weizmann Institute of Science, Rehovot, Israel, September 19, 2008 (received for review May 15, 2008)

Abstract

Intrinsically disordered proteins play critical but often poorly understood roles in mediating protein interactions. The interactions of disordered proteins studied to date typically entail structural stabilization, whether as a global disorder-to-order transition or minimal ordering of short linear motifs. The disordered cyclin-dependent kinase (CDK) inhibitor Sic1 interacts with a single site on its receptor Cdc4 only upon phosphorylation of its multiple dispersed CDK sites. The molecular basis for this multisite-dependent interaction with a single receptor site is not known. By NMR analysis, we show that multiple phosphorylated sites on Sic1 interact with Cdc4 in dynamic equilibrium with only local ordering around each site. Regardless of phosphorylation status, Sic1 exists in an intrinsically disordered state but is surprisingly compact with transient structure. The observation of this unusual binding mode between Sic1 and Cdc4 extends the understanding of protein interactions from predominantly static complexes to include dynamic ensembles of intrinsically disordered states.

Footnotes

³To whom correspondence may be addressed at:
Centre for Systems Biology, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue, Toronto, ON, Canada M5G 1X5.
E-mail: tyers{at}mshri.on.ca
⁴To whom correspondence may be addressed at:
Molecular Structure and Function, Hospital for Sick Children, 555 University Avenue, Toronto, ON, Canada M5G 1X8.
E-mail: forman{at}sickkids.ca
Author contributions: T.M., S.O., W.-Y.C., X.T., F.S., L.E.K., M.T., and J.D.F.-K. designed research; T.M., S.O., W.-Y.C., X.T., and H.L. performed research; T.M., S.O., W.-Y.C., X.T., L.E.K., and J.D.F.-K. analyzed data; and T.M., M.T., and J.D.F.-K. wrote the paper.
↵¹Present address: Department of Biochemistry, University of Western Ontario, London, ON, Canada N6A 5C1.
↵²Present address: Wellcome Trust Centre for Cell Biology, School of Biological Sciences, University of Edinburgh, Edinburgh Eh9 3Jr, Scotland.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0809222105/DCSupplemental.
Freely available online through the PNAS open access option.