Two different neurodegenerative diseases caused by proteins with similar structures

Abstract

The downstream prion-like protein (doppel, or Dpl) is a paralog of the cellular prion protein, PrP(C). The two proteins have approximately 25% sequence identity, but seem to have distinct physiologic roles. Unlike PrP(C), Dpl does not support prion replication; instead, overexpression of Dpl in the brain seems to cause a completely different neurodegenerative disease. We report the solution structure of a fragment of recombinant mouse Dpl (residues 26-157) containing a globular domain with three helices and a small amount of beta-structure. Overall, the topology of Dpl is very similar to that of PrP(C). Significant differences include a marked kink in one of the helices in Dpl, and a different orientation of the two short beta-strands. Although the two proteins most likely arose through duplication of a single ancestral gene, the relationship is now so distant that only the structures retain similarity; the functions have diversified along with the sequence.

Figure 1

Alignment of the amino acid sequences of mouse Dpl (19) and mouse PrP^C (46). The sequences of the 26–157 construct of Dpl used in the present work, and of the 23–231 construct of mouse PrP^C (47), are shown in black letters. Blue letters compose the signal sequence at the N terminus and the C-terminal hydrophobic region. The octarepeat sequence of PrP^C (not present in Dpl) has been omitted for brevity. The pink bar indicates identical amino acids. Red and blue boxes indicate the location of α and β secondary structures in Dpl (this work) and mouse PrP^C (3). Helices are labeled αA, αB, and αC; the location of the kink in the αB helix is shown. αB′ indicates the B helix C-terminal to the kink. The dashed red box at residues 220–227 of PrP^C shows the location of the extended αC helix in Syrian hamster (4), bovine (5), and human (6) PrP^C.

Figure 2

¹H-¹⁵N HSQC spectrum of Dpl(26–157) showing resonance assignments. Insets show assignments for the crowded central region (Left) and for the Gln and Asn side-chain amides (Right). Cross peaks for Gly-89 and Glu-93 are folded to the opposite side of the spectrum.

Figure 3

Solution structure of mouse Dpl. (a) Stereo representation of the backbone of a family of 20-solution structures of Dpl(26–157) showing residues 51–157. The N and C termini are labeled, and the positions of the helices αA, αB, and αC are indicated, with the portion of the α-B helix C-terminal to the kink labeled αB′. The positions of the two disulfide bonds in one of the structures are shown in yellow. (b) Comparison of the backbone topology of mouse Dpl(26–157), mouse PrP(121–231) (3), and Syrian hamster PrP(90–231) (4).

Figure 4

Plot of the heteronuclear {¹H}-¹⁵N NOE values observed for each residue in Dpl(26–157). The positions of secondary structure elements are denoted by labeled black bars for helices and gray bars for the two β-strands.