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Intrinsically unfolded proteins (IUPs) do not obey the golden rule of structural biology, 3D structure = function, as they manifest their inherent functions without resorting to three-dimensional structures. Absence of a compact globular topology in these proteins strongly implies that their ligand recognition processes should involve factors other than spatially well-defined binding pockets. Heteronuclear multidimensional (HetMulD) NMR spectroscopy assisted with a stable isotope labeling technology is a powerful tool for quantitatively investigating detailed structural features in IUPs. In particular, it allows us to delineate the presence and locations of pre-structured motifs (PreSMos) on a per-residue basis. PreSMos are the transient local structural elements that presage target-bound conformations and act as specificity determinants for IUP recognition by target proteins. Here, we present a brief chronicle of HetMulD NMR studies on IUPs carried out over the past two decades along with a discussion on the functional significance of PreSMos in IUPs.

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p53 is an important tumor-suppressor protein deactivation of which by mdm2 results in cancers. A SUMO-specific protease 4 (SUSP4) was shown to rescue p53 from mdm2-mediated deactivation, but the mechanism is unknown. The discovery by NMR spectroscopy of a "p53 rescue motif" in SUSP4 that disrupts p53-mdm2 binding is presented. This 29-residue motif is pre-populated with two transient helices connected by a hydrophobic linker. The helix at the C-terminus binds to the well-known p53-binding pocket in mdm2 whereas the N-terminal helix serves as an affinity enhancer. The hydrophobic linker binds to a previously unidentified hydrophobic crevice in mdm2. Overall, SUSP4 appears to use two synergizing modules, the p53 rescue motif described here and a globular-structured SUMO-binding catalytic domain, to stabilize p53. A p53 rescue motif peptide exhibits an anti-tumor activity in cancer cell lines expressing wild-type p53. A pre-structures motif in the intrinsically disordered proteins is thus important for target recognition.