**1. Introduction**

Prion diseases or transmissible spongiform encephalopathies (TSEs) are rare neurodegenerative diseases exhibiting symptoms of both cognitive and motor dysfunction, vacuolation of the grey matter in the human central nervous system, neuronal loss, and astrogliosis [1]. A crucial event for the diseases' development is the misfolding of the extracellular, membrane-anchored human prion protein (HuPrPC) into the fibril-forming isoform called "scrapie" (HuPrPSc), the major or only component of the infectious particle [2]. This eventually leads to protofibril and fibrillar structures. Accordingly, with the "Protein only hypothesis" by Nobel Laureate Prusiner [3], the feature to undergo induced or spontaneous misfolding depends basically on intrinsic features of the protein. These include the amino acid sequence [4,5] as well as secondary structure elements [6–8], the highly flexible amino terminal region of the protein [9], and posttranslational modification elements [10]. The propensity to form the scrapie form is modulated by a variety of external factors. These include pH [11–13], cofactors like metal ions [14,15], or the presence of proteins [16,17]. Pathogenic mutations (PM) in HuPrP<sup>C</sup> are linked to the spontaneous generation of prion diseases [18–21].

HuPrPC is ubiquitously expressed throughout the body. It is mostly found in the central nervous system. After being synthesized in the rough endoplasmic reticulum, it transits through the Golgi compartment, and it is released to the cell surface where it resides in lipid membrane domains [22]. Though its physiological role is still not clear, HuPrPC might be involved in neuronal development,

cell adhesion, apoptotic events, and cell signaling in the central nervous system. Moreover, HuPrP<sup>C</sup> can interact with different neuronal proteins or proteins of the extracellular matrix, as well as with other binders including glycosaminoglycans, nucleic acids, and copper ions [23]. Hence, HuPrP<sup>C</sup> has been also proposed as a copper sensing or transport protein [24].

The protein features two signal peptides (1–22 and 232–235, Figure 1), a folded globular domain (GD, residues 125–231), and a naturally unfolded N-terminal tail (N-term\_HuPrPC, hereafter, residues 23–124), which is the focus of this review. The GD consists of two β-sheets (S1 and S2), three α-helices (H1, H2, and H3), one disulfide bond (SS) between cysteine residues 179 and 214, and two potential sites for N-linked glycosylation (green forks in Figure 1) at residues 181 and 197 [25]. H2 and H3 helices linked by the SS-bond constitute the H2 + H3 domain. A glycosylphosphatidylinositol anchor (GPI, in blue in Figure 1) is attached to the C-terminus, which is located on the outside cellular membrane.

**Figure 1.** (**A**) Schematic and (**B**) tridimensional view of HuPrPC. (**C**) Qualitative scheme illustrating the Gibbs free energy change in the conversion from HuPrP<sup>C</sup> (left) to HuPrPSc (right) [26]. The depicted amyloidogenic intermediate is the parallel, in-register β-structure model for the core of recombinant PrP90–231 amyloid fibrils formed in vitro [27], one of the models among others [28–30], whereas the native globular domain (GD) of the HuPrP<sup>C</sup> is the nuclear magnetic resonance (NMR) structure by Zahn et al. [25]. Adapted from [31,32].

The HuPrPC→HuPrPSc interconversion involves mostly the GD. It may entail increasingly β-stranded intermediate structures [33] (Figure 1C), leading to small aggregates, protofibrils, and finally ordered rigid fibrils [34–38]. Experimental structural information for these is lacking [34–38].

While the structure of the GD of HuPrP<sup>C</sup> has been resolved experimentally, the intrinsically disordered nature of the N-term\_HuPrP<sup>C</sup> has represented a challenge for structural studies. In this paper, the structural properties of the N-term\_HuPrPC are discussed, with a focus on recent insights obtained from computational approaches and on the functional and disease-related implications of copper–N-term\_HuPrPC interactions.
