The chromatin nuclear protein NUPR1L is intrinsically disordered and binds to the same proteins as its paralogue

NUPR1 is a protumoral multifunctional intrinsically disordered protein (IDP), which is activated during the acute phases of pancreatitis. It interacts with other IDPs such as prothymosin α, as well as with folded proteins such as the C-terminal region of RING1-B (C-RING1B) of the Polycomb complex; in all those interactions, residues around Ala33 and Thr68 (the ‘hot-spot’ region) of NUPR1 intervene. Its paralogue, NUPR1L, is also expressed in response to DNA damage, it is p53-regulated, and its expression down-regulates that of the NUPR1 gene. In this work, we characterized the conformational preferences of isolated NUPR1L and its possible interactions with the same molecular partners of NUPR1. Our results show that NUPR1L was an oligomeric IDP from pH 2.0 to 12.0, as judged by steady-state fluorescence, circular dichroism (CD), dynamic light scattering, 1D ¹H-NMR (nuclear magnetic resonance), and as indicated by structural modelling. However, in contrast with NUPR1, there was evidence of local helical- or turn-like structures; these structures were not rigid, as judged by the lack of sigmoidal behaviour in the chemical and thermal denaturation curves obtained by CD and fluorescence. Interestingly enough, NUPR1L interacted with prothymosin α and C-RING1B, and with a similar affinity to that of NUPR1 (in the low micromolar range). Moreover, NUPR1L hetero-associated with NUPR1 with an affinity of 0.4 µM and interacted with the ‘hot-spot’ region of NUPR1. Thus, we suggest that the regulation of NUPR1 gene by NUPR1L does not only happen at the DNA level, but it could also involve direct interactions with NUPR1 natural partners.