50% increase; Fig. 6a and Suppl. Fig. 9b), consistent with partial displacement of p27-D2 from Cdk2/cyclin A by SJ403. Furthermore, in the absence of p27-D2, SJ403 substantially inhibited Cdk2/cyclin A activity at concentrations that, in the presence of p27-D2, were associated with p27-D2 displacement and increased kinase activity (Fig. 6b and Suppl. 9c). Thus, we conclude that the primary effect of SJ403 is displacement of p27-D2 from Cdk2/cyclin A (through the binding of SJ403 to p27-D2) and partial restoration of kinase activity, even as a secondary effect of SJ403 is to inhibit kinase activity (through the binding of SJ403 to Cdk2/cyclin A). These results provide proof-of-principle that a small molecule (SJ403) inhibits the function of a disordered protein (p27-D2) through sequestration in a conformation incapable of binding and inhibiting Cdk2/cyclin A./p>20 Å) and that Group 2 molecules bind to compact conformations when at least two of the three critical aromatic residues within the different sub-regions (sub-domains D2.1, D2.2 and D2.3) are clustered. Interestingly, analysis of the MD trajectory showed that Y88 and either W60 or W76 were frequently in close contact but that all three residues were rarely in close proximity (Suppl. Fig. 10c). This suggested that there are several different conformations with clustered aromatic residues (in particular, Y88 and either W60 or W76) capable of binding to Group 2 compounds, consistent with mutagenesis results showing that either W60 or W76, but not both, are dispensable for Group 2 compound binding (Suppl. Fig. 5a–f). In summary, the new MD results for p27-D2 suggest strongly that transient conformational fluctuations that create and disrupt clusters of aromatic residues modulate the binding of Group 1 and Group 2 small molecules to p27-D2. These results are consistent with the identification of W60, W76 and Y88 by NMR as the principal sites for compound binding and with results showing that binding is altered through mutation of these residues./p>2,300 compounds that were screened failed to bind other regions of p27 (sub-domains D1 and LH), suggesting that these other regions lack a sufficient density of aromatic residues (specifically, W and Y residues) to specifically recognize small heterocyclic aromatic molecules. Sub-domain LH, in isolation, does not bind to Cdk2/cyclin A but sub-domain D1 binds cyclin A with high affinity (Kd, 42 nM)45. We speculate that the RxLFG motif within this latter region, due to its limited length, cannot adopt conformations that create binding pockets for small molecules, as is possible for the much longer D2 sub-domain. However, the low affinity of the Group 1 and 2 compounds for p27 limits their applicability toward our broader goal of modulating p27 function in cells and, ultimately, humans. How can the affinity of small molecules for p27 be increased? We propose that the Group 1 and 2 molecules cause a degree of conformational restriction within p27-D2 and that molecules that enhance this restriction will exhibit higher affinity. We envision that small molecules with greater "three-dimensionality", that present chemically diverse and complex features, will be better templates for binding and sequestering p27. Efforts based on two strategies are underway to optimize our fragment hits using synthetic chemistry. First, we are "growing" the Group 1 and 2 scaffolds by introducing diverse chemical moieties at various positions on the heterocyclic ring systems to enable additional interactions with residues near W60, W76 and Y88 within p27-D2. Second, when the growing experiments are complete, we will synthetically "link" the optimal Group 1 and Group 2 molecules to further enhance binding to p27-D2. The results of these future experiments will indicate whether synthetic strategies for compound optimization that have emerged from structure-based drug discovery can be applied to a disordered protein. In conclusion, we have discovered small molecules with "fuzzy SAR" that mediate specific binding to and inhibition of p27, demonstrating the potential to rationally "drug" disordered protein targets in the future./p>500,000 compounds; see below). First, commercial fragment collections (subsets of larger diversity collections filtered for ‘fragment-like’ characteristics) were filtered to remove molecules containing inorganic atoms, isotopes, or invalid structures and to remove molecules that were not available in sufficient quantity (<50 mg). Passing molecules were abstracted to Murcko scaffolds using Pipeline Pilot (‘Generate Fragments’ component in Accelrys v. 8.5 with alpha atoms preserved, see ref. 48 for the general method). These scaffolds were further filtered according to the following rules: number of reactive substructures = 0 (‘REOS’ filters49,50,51, number of rotatable bonds <= 3, number of heavy atoms >= 10, number of rings >= 1 and number of ring substitutions >1 for single ring systems and number of molecules present in the St. Jude HTS library containing the scaffold >= 8. Molecules containing these scaffolds were identified in the commercial fragment libraries and then prioritized for purchase according to highest Oprea complexity52. This library has the following average calculated physicochemical properties: MW = 246 ± 39 Da, number of atoms = 17 ± 3, log P = 1.7 ± 1.0, polar surface area = 63 ± 19 Å2, number of H-bond acceptors = 4.3 ± 1.4 and number of H-bond donors = 1.3 ± 0.9. The distributions of these and other chemical features of the two fragment libraries are summarized in Suppl. Fig. 12a./p>3.0.CO;2-Q" data-track-action="article reference" href="https://doi.org/10.1002%2F%28SICI%291521-3773%2819990614%2938%3A12%3C1784%3A%3AAID-ANIE1784%3E3.0.CO%3B2-Q" aria-label="Article reference 36" data-doi="10.1002/(SICI)1521-3773(19990614)38:123.0.CO;2-Q"Article CAS Google Scholar /p>