Antagonistic roles of the N-terminal domain of prion protein to doppel

Abstract

Prion protein (PrP)-like molecule, doppel (Dpl), is neurotoxic in mice, causing Purkinje cell degeneration. In contrast, PrP antagonizes Dpl in trans, rescuing mice from Purkinje cell death. We have previously shown that PrP with deletion of the N-terminal residues 23-88 failed to neutralize Dpl in mice, indicating that the N-terminal region, particularly that including residues 23-88, may have trans-protective activity against Dpl. Interestingly, PrP with deletion elongated to residues 121 or 134 in the N-terminal region was shown to be similarly neurotoxic to Dpl, indicating that the PrP C-terminal region may have toxicity which is normally prevented by the N-terminal domain in cis. We recently investigated further roles for the N-terminal region of PrP in antagonistic interactions with Dpl by producing three different types of transgenic mice. These mice expressed PrP with deletion of residues 25-50 or 51-90, or a fusion protein of the N-terminal region of PrP with Dpl. Here, we discuss a possible model for the antagonistic interaction between PrP and Dpl.

Figure 1

(A) Schemes of wild-type mouse PrP and Dpl. Mouse PrP is first translated as a precursor protein consisting of 254 amino acids. The N-terminal 22 and C-terminal 23 hydrophobic amino acids are removed as a signal peptide and a GPI-anchor signal sequence, respectively. The N-terminal half of PrP^C is highly flexible and lacks identifiable secondary structure. The octapeptide repeat (OR) region, comprising five copies of a P(H/Q)GGG(G)WGQ octapeptide sequence, is located in the N-terminal domain. The OR region is thought to mediate anti-oxidative activity by binding to Cu²⁺ via histidine residues. However, the exact function of this region remains to be elucidated. The C-terminal half of PrP^C forms a globular structure with three α-helices (α1–3) and two short anti-parallel β-strands (β1, β2). The second and third helices are linked by a disulfide bond (-S-S-). The precursor protein of Dpl consists of 179 amino acids. The N-terminal 25 and C-terminal 22 hydrophobic residues may be removed as signal peptide and GPI-anchor signals, respectively. Dpl is a structural homologue of the C-terminal globular domain of PrP^C, sharing ∼23% identical amino acids and is composed of three α-helices (α1–3) and two short anti-parallel β-strands (β1, β2). Two disulfide bonds (-S-S-) are formed. However, Dpl lacks the corresponding N-terminal part of PrP^C. (B) Structural schemes of PrPs with deletion of various regions and PrPN-Dpl, the fusion protein composed of the N-terminal region of PrP with Dpl, with their cis- and trans-protective activity against Dpl or toxic PrPs are shown. a: Construct 7 is itself non-toxic. However, it has different affects on neurotoxic Constructs 2 and 6: It enhances the toxicity of Construct 6 but diminishes that of Construct 2. NA: data are not available.

Figure 2

A possible mechanism for the antagonistic interaction of PrP^C and Dpl or the toxic PrPs. (A) Dpl binds to a putative transmembrane molecule, producing a toxic signal. Toxic PrPs with deletion of the central region, such as Constructs 1, 2, 5 and 6, bind to the transmembrane molecule via the Dpl-homologous C-terminal area in the same way as Dpl, eliciting a similar toxic signal. (B) Under normal conditions, wild-type PrP^C binds to the trans-membrane molecule via the N-terminal region but not its C-terminal region because it forms either a homo-dimer linked via the central region or a monomer with the central region interacting with part of the C-terminal domain. The N-terminal region acquires binding affinity to the molecule only when the central region is intact. However, this type of interaction produces no toxic signal. (C) PrPs with part of the N-terminal region and with the central region both intact, such as trans- protective PrPs, have a higher affinity for the transmembrane molecule than Dpl or the toxic PrPs, resulting in trans-protection against Dpl and the toxic PrPs. (D) Construct 4 (PrPΔ23–88) still has potential to form a homo-dimer due to the residual central region or a monomer with the residual central region masking part of the C-terminal region, similarly to wild-type PrP^C. Therefore, Construct 4 cannot form a complex with the transmembrane molecule via the C-terminal region, generating no toxic signal. In addition, by lacking part of the N-terminal domain, Construct 4 has no affinity for the transmembrane molecule, losing trans-protective activity against Dpl.