Beta structure motifs of islet amyloid polypeptides identified through surface-mediated assemblies

Abstract

We report here the identification of the key sites for the beta structure motifs of the islet amyloid polypeptide (IAPP) analogs by using scanning tunneling microscopy (STM). Duplex folding structures in human IAPP(8-37) (hIAPP(8-37)) assembly were observed featuring a hairpin structure. The multiplicity in rIAPP assembly structures indicates the polydispersity of the rat IAPP(8-37) (rIAPP(8-37)) beta-like motifs. The bimodal length distribution of beta structure motifs for rIAPP(8-37) R18H indicates the multiple beta segments linked by turns. The IAPP(8-37) analogs share common structure motifs of IAPP(8-17) and IAPP(26-37) with the most probable key sites at positions around Ser(19)/Ser(20) and Gly(24). These observations reveal the similar amyloid formation tendency in the C and N terminus segments because of the sequence similarity, while the differences in specific amino acids at each key site manifest the effect of sequence variations. The results could be beneficial for studying structural polymorphism of amyloidal peptides with multiple beta structure motifs.

Fig. 1.

Molecular structure of the chaperon-like molecule 4,4′-bipyridyl (4Bpy) and the sequences of hIAPP_8–37, hIAPP_37–8, rIAPP_8–37, rIAPP_37–8, rIAPP_8–37 R18H, and rIAPP_37–8 R18H. The residues in rIAPP_8–37, rIAPP_37–8, rIAPP_8–37 R18H, and rIAPP_37–8 R18H that differ from hIAPP_8–37 and hIAPP_37–8 are depicted in red, and the corresponding positions in hIAPP_8–37 and hIAPP_37–8 are depicted in blue. The residues in rIAPP_8–37 R18H and rIAPP_37–8 R18H depicted in blue are the mutation sites His₁₈. Color code: cyan for C, gray for H, yellow for N, and pink for O.

Fig. 2.

High resolution STM image of hIAPP_8–37 assembly. The length of the yellow bar, covering 10 molecules, is 4.7 nm. The molecular axes of hIAPP_8–37 peptides indicated by a group of short white lines are perpendicular to the long axis of the stripe (highlighted by the red line). Tunneling conditions: I = 350.0 pA, V = 490.0 mV.

Fig. 3.

(A and D) High resolution STM images of hIAPP_8–37 (A) and hIAPP_37–8 (D) coassembled with 4Bpy. The angles α between peptide molecular axes and the stripe directions are measured to be 34 ± 2° both in A and D. Tunneling conditions: (A) I = 314.3 pA, V = 605.2 mV; (D) I = 532.8 pA, V = 379.7 mV. Inset in D is large scale STM image of hIAPP_37–8/4Bpy. (B and E) Length distribution histograms of beta-structure motifs for hIAPP_8–37 and hIAPP_37–8, respectively, measured from the STM images. The step size of the chart is 0.325 nm and is equivalent to the separation of two neighboring residues in the parallel beta-sheet structure. (C and F) Proposed models for the folding sites in hIAPP_8–37 and hIAPP_37–8 beta-structure motifs, respectively. Red rings highlight the folding sites in beta-structure motif. The green belts stand for the beta-structure motifs, and the flexible red strings for the undiscernible parts in STM observations. Color code: green for C, gray for H, blue for N, and red for O. The side-chain conformations are shown in schematic way. The crystalline structure of hIAPP_21–27 and hIAPP_28–33 are referred to for the proposal of this schematic illustration (15).

Fig. 4.

(A and D) High resolution STM images of rIAPP_8–37 (A) and rIAPP_37–8 (D) coassembled with 4Bpy. The angles α between peptide molecular axes and the stripe directions are measured to be 36 ± 2° (A) and 30 ± 2° (D). Tunneling conditions: (A) I = 550.1 pA, V = 322.8 mV; (D) I = 611.5 pA, V = 300.0 mV. Insets in A and D are large scale STM images. (B and E) Length distribution histograms of rIAPP_8–37 (B) and rIAPP_37–8 (E) beta-structure motifs measured from the STM images. The step sizes for the histograms are also 0.325 nm, which is the same as Fig. 3. (C and F) Proposed models for the folding sites in rIAPP_8–37 and rIAPP_37–8 beta-structure motifs respectively. The denotation for the colors and the shapes are the same as Fig. 3.

Fig. 5.

(A and D) High resolution STM images of rIAPP_8–37 R18H (A) and rIAPP_37–8 R18H (D) coassembled with 4Bpy. The angles (α) between peptide molecular axes and the stripe directions are measured to be 34 ± 2° (A) and 36 ± 2° (D). Tunneling conditions: (A) I = 612.9 pA, V = 300.4 mV; (D) I = 575.9 pA, V = 334.4 mV. Insets in A and D are large scale STM images. (B and E) Length distribution histograms of rIAPP_8–37 R18H (B) and hIAPP_37–8 R18H (E) beta-structure motifs measured from the STM images. The step sizes for the histograms are also 0.325 nm, which is the same as Fig. 3. (C and F) Proposed models for the folding sites in rIAPP_8–37 R18H and rIAPP_37–8 R18H beta-structure motifs respectively. The denotation for the colors and the shapes are the same as Fig. 3.