Human parathyroid hormone (PTH), comprising 84 amino acids, is secreted by the parathyroid glands and plays a pivotal role in regulating serum phosphate and calcium levels in the blood by acting on the bone, kidney, and intestines. PTH is also counted as a functional amyloid due to its proposed ability to store itself in the form of amyloids prior to its release. The classical actions of PTH are mediated through binding of the N-terminal domain, constituted by the first 34 amino acids, to the PTH receptor present in many tissues. However, the role of intrinsically disordered C-terminal domain remains less understood. In this study, we investigated the role of the intrinsically disordered C-terminal region on pre-fibrillar aggregates by comparing sizes, distribution, and structures of oligomers of the truncated variant and the full length protein using spectroscopic techniques as fluorescence correlation spectroscopy (FCS), single-molecule fluorescence resonance energy transfer (smFRET), single-molecule fluorescence spectroscopy (smFS), and infrared (IR) spectroscopy, while fibrillation kinetics were monitored with thioflavin T (ThT) fluorescence assays. Our findings reveal that the absence of the C-terminal region leads to smaller oligomer and nuclei sizes as well as to accelerated fibrillation. In contrast, the full-length peptide hormone forms larger pre-fibrillar assemblies and retards fibrillation. These results provide molecular insight into how disordered region regulate the dynamics of functional amyloids. Moreover, using macromolecular crowding conditions, we could identify the differences in fibrillation kinetics, sizes, and structural changes to the compensatory effect of the intrinsically disordered region.
Bhatia et al. (Sun,) studied this question.