Post-translational modifications and conformational transitions of the intrinsically disordered oncoproteins High-Mobility Group A

Intrinsically disordered proteins (IDPs) are flexible molecules, able to adapt to the surfaces of different molecular partners by means of specific, but easily reversible interactions. IDPs carry out pivotal biological functions participating in almost all cell signaling and regulatory pathways. Importantly, IDPs activities are finely modulated by the addition/removal of numerous post-translational modifications (PTMs) which are important conformational modulators. Prototypes of IDPs are High Mobility Group A (HMGA) proteins, which are expressed at high levels and play essential functions both in embryonic and cancer cells. HMGA protein family (HMGA1a, HMGA1b, and HMGA2) belong to the non-histone HMG chromatin protein super-family and are multifunctional architectural transcription factors. HMGA conformational adaptability and intricate pattern of dynamic and constitutive PTMs are thought to be responsible for this multifunctionality. We performed a liquid chromatography-mass spectrometry (LC-MS) screening in twenty different cell lines in order to evaluate HMGA proteins PTM pattern and we evidenced relevant intra-family differences. Moreover, we focused on the poorly characterized HMGA2 and we mapped HMGA2 phosphorylation sites by mass sequencing demonstrating that, similarly to HMGA1, it is phosphorylated on the acidic C-terminal tail by CK2. Importantly, this modification turned out to affect HMGA2 DNA binding. Since truncated HMGA proteins are more oncogenic than full-length ones and since HMGA are in vivo heavily modified on their C-terminal domain, we dissected the role of this domain and its phosphorylations from a structural point of view. We probed HMGA IDPs compactness and accessibility taking advantage of an innovative approach combining limited proteolysis and MS-based techniques. By limited proteolysis, ESI (electrospray ionization)-MS, and IMS (ion mobility separation)-MS we demonstrated that HMGA can assume a compact conformation and that their compactness degree is dependent upon the presence of the acidic C-terminal domain and its constitutive phosphorylations. Moreover, LC-MS analyses after enzymatic assays showed that HMGA forms with a deletion of acidic C-terminal tail are more susceptible to PTMs, thus supporting the idea that the acidic tail is involved in masking the accessibility of modifying enzymes to their own consensus sites. We evidenced macroscopic differences regarding PTMs affecting the three HMGA family members and provided the first data about in vivo HMGA2 PTMs and their effect on DNA binding. Our structural investigations revealed a structure/PTMs relationship dictated by the presence of the C-terminal domain. This evidence, together with the already known in vivo functional outcome of HMGA C-terminal truncation, suggests a structure/function link between HMGA tails, their PTMs, and their oncogenic properties, paving the way for the development of interfering therapeutic strategies based on targeting HMGA proteins.