Probing the N‑Terminal β‑Sheet Conversion in the Crystal Structure of the Human Prion Protein Bound to a Nanobody

Prions are fatal neurodegenerative
transmissible agents causing
several incurable illnesses in humans and animals. Prion diseases
are caused by the structural conversion of the cellular prion protein,
PrPC, into its misfolded oligomeric form, known as prion
or PrPSc. The canonical human PrPC (HuPrP) fold
features an unstructured N-terminal part (residues 23–124)
and a well-defined C-terminal globular domain (residues 125–231).
Compelling evidence indicates that an evolutionary N-terminal conserved
motif AGAAAAGA (residues 113–120) plays an important role in
the conversion to PrPSc. The intrinsic flexibility of the
N-terminal has hampered efforts to obtain detailed atomic information
on the structural features of this palindromic region. In this study,
we crystallized the full-length HuPrP in complex with a nanobody (Nb484)
that inhibits prion propagation. In the complex, the prion protein
is unstructured from residue 23 to 116. The palindromic motif adopts
a stable and fully extended configuration to form a three-stranded
antiparallel β-sheet with the β1 and β2 strands,
demonstrating that the full-length HuPrPC can adopt a more
elaborate β0-β1-α1-β2-α2-α3 structural
organization than the canonical β1-α1-β2-α2-α3
prion-like fold. From this structure, it appears that the palindromic
motif mediates β-enrichment in the PrPC monomer as
one of the early events in the conversion of PrPC into
PrPSc.

朊病毒（Prions）是一类致命的可传播神经退行性病原体，可引发人类与动物罹患多种无法治愈的疾病。朊病毒病（Prion diseases）由细胞型朊蛋白（cellular prion protein, PrPC）发生结构转化，转变为错误折叠的寡聚体形式（即朊病毒或PrPSc）所导致。经典型人源细胞型朊蛋白（HuPrP）的折叠结构包含一段无结构化的N端区域（残基23–124）与一个结构明确的C端球状结构域（残基125–231）。大量有力证据表明，一段进化保守的N端基序AGAAAAGA（残基113–120）在向PrPSc的转化过程中发挥重要作用。N端区域的内在柔性阻碍了研究者获取该回文区域结构特征的高精度原子级信息。本研究中，我们成功解析了全长人源细胞型朊蛋白与抑制朊病毒增殖的纳米抗体（Nb484）形成的复合物晶体结构。在该复合物中，朊蛋白从23位至116位残基处于无结构化状态。该回文基序呈现稳定的完全伸展构象，与β1、β2链共同形成三股反平行β折叠片层，这表明全长人源细胞型朊蛋白可形成比经典β1-α1-β2-α2-α3类朊蛋白折叠更为复杂的β0-β1-α1-β2-α2-α3结构排布。基于该晶体结构，回文基序可能介导了PrPC单体的β折叠富集，这或许是PrPC向PrPSc转化过程中的早期事件之一。