Prostate-associated Gene-4 (PAGE4) is an intrinsically disordered Cancer/Testis Antigen that is up-regulated in the fetal and diseased human prostate. Knocking down PAGE4 expression results in cell death while its over-expression leads to a growth advantage of prostate cancer cells [Zeng et al. (2011) J. Biol. Chem. 286, 13985-13994]. Phosphorylation of PAGE4 at T51 is critical for potentiating c-Jun transactivation, an important factor in controlling cell growth, apoptosis, and stress response. Using NMR spectroscopy, we show that the PAGE4 polypeptide chain has local and long-range conformational preferences that are perturbed by site-specific phosphorylation at T51. The population of transient turn-like structures increases upon phosphorylation in an approximately 20-residue acidic region centered on T51. This central region thus becomes more compact and more negatively charged, with increasing intramolecular contacts to basic sequence motifs near the N- and C-termini. While flexibility is decreased in the central region of phospho-PAGE4, the polypeptide chain remains highly dynamic overall. PAGE4 utilizes a transient helical structure adjacent to the central acidic region to bind c-Jun with low affinity in vitro. The binding interaction is attenuated by phosphorylation at T51, most likely due to masking effects of the more compact phosphorylated state. Thus, phosphorylation of PAGE4 leads to conformational shifts in the dynamic ensemble with large functional consequences. The changes in the structural ensemble induced by post-translational modifications are similar conceptually to the conformational switching events seen in some marginally stable (metamorphic) folded proteins in response to mutation or environmental triggers.

Intrinsically disordered proteins, IDPs, are proteins that lack a rigid 3D structure under physiological conditions, at least in vitro. Despite the lack of structure, IDPs play important roles in biological processes and transition from disorder to order upon binding to their targets. With multiple conformational states and rapid conformational dynamics, they engage in myriad and often “promiscuous” interactions. These stochastic interactions between IDPs and their partners, defined here as conformational noise, is an inherent characteristic of IDP interactions. The collective effect of conformational noise is an ensemble of protein network configurations, from which the most suitable can be explored in response to perturbations, conferring protein networks with remarkable flexibility and resilience. Moreover, the ubiquitous presence of IDPs as transcriptional factors and, more generally, as hubs in protein networks, is indicative of their role in propagation of transcriptional (genetic) noise. As effectors of transcriptional and conformational noise, IDPs rewire protein networks and unmask latent interactions in response to perturbations. Thus, noise-driven activation of latent pathways could underlie state-switching events such as cellular transformation in cancer. To test this hypothesis, we created a model of a protein network with the topological characteristics of a cancer protein network and tested its response to a perturbation in presence of IDP hubs and conformational noise. Because numerous IDPs are found to be epigenetic modifiers and chromatin remodelers, we hypothesize that they could further channel noise into stable, heritable genotypic changes.