Mechanistic studies of peptide self-assembly: transient α-helices to stable β-sheets.
The pathologic self-assembly of proteins is associated with typically late-onset disorders such as Alzheimer's disease, Parkinson's disease, and type 2 diabetes. Important mechanistic details of the self-assembly are unknown, but there is increasing evidence supporting the role of transient α-helices in the early events. Islet amyloid polypeptide (IAPP) is a 37-residue polypeptide that self-assembles into aggregates that are toxic to the insulin-producing β cells. To elucidate early events in the self-assembly of IAPP, we used limited proteolysis to identify an exposed and flexible region in IAPP monomer. This region includes position 20 where a serine-to-glycine substitution (S20G) is associated with enhanced formation of amyloid fibrils and early onset type 2 diabetes. To perform detailed biophysical studies of the exposed and flexible region, we synthesized three peptides including IAPP(11-25)WT (wild type), IAPP(11-25)S20G, and IAPP(11-25)S20P. Solution-state NMR shows that all three peptides transiently populate the α-helical conformational space, but the S20P peptide, which does not self-assemble, transiently samples a broken helix. Under similar sample conditions, the WT and S20G peptides populate the α-helical intermediate state and β-sheet end state, respectively, of fibril formation. Our results suggest a mechanism for self-assembly that includes the stabilization of transient α-helices through the formation of NMR-invisible helical intermediates followed by an α-helix to β-sheet conformational rearrangement. Furthermore, our results suggest that reducing intermolecular helix-helix contacts as in the S20P peptide is an attractive strategy for the design of blockers of peptide self-assembly.
Carlson School of Chemistry and Biochemistry, Clark University, 950 Main Street, Worcester, Massachusetts 01610, USA.
Gai
G
Liu
Anabathula
A
Prabhakar
Darryl
D
Aucoin
Miranda
M
Simon
Samuel
S
Sparks
Kevin J
KJ
Robbins
Andrew
A
Sheen
Sarah A
SA
Petty
Noel D
ND
Lazo
D019943:Amino Acid Substitution; D005998:Glycine / Q000737:chemistry; D006801:Humans; D058228:Islet Amyloid Polypeptide / Q000037:antagonists & inhibitors / Q000138:chemical synthesis / Q000737:chemistry*; D019906:Nuclear Magnetic Resonance, Biomolecular; D011392:Proline / Q000737:chemistry; D017433:Protein Structure, Secondary; D012694:Serine / Q000737:chemistry
D058228:Islet Amyloid Polypeptide; D012694:Serine; D011392:Proline; D005998:Glycine