Analysis of Proline Substitutions Reveals the Plasticity and Sequence Sensitivity of Human IAPP Amyloidogenicity and Toxicity.

Pancreatic amyloid formation by the polypeptide IAPP contributes to -cell dysfunction in type 2 diabetes. There is a one-to-one correspondence between the ability of IAPP from different species to form amyloid in vitro and the susceptibility of the organism to develop diabetes. Rat IAPP is non-amyloidogenic and differs from human IAPP at six positions, including three proline replacements: A25P, S28P, S29P. Incorporation of these proline residues into human IAPP leads to a non-amyloidogenic analogue which is used clinically. The role of the individual proline residues is not understood. We examine the three single and three double proline substitutions in the context of human IAPP. An S28P substitution significantly decreases amyloidogenicity and toxicity, while an S29P substitution has very modest effects despite being an identical replacement just one residue away. The consequences of the A25P substitution are between those of the two Ser to Pro substitutions. Double mutants containing an S28P replacement are less amyloidogenic and less toxic than the IAPPA25P S29P double mutant. Ion mobility mass spectrometry reveals that there is no correlation between monomer or dimer conformation as reported by collision cross sections measurements and the time to form amyloid. The work reveals both the plasticity of IAPP amyloid formation and the exquisite sequence sensitivity of IAPP amyloidogenicity and toxicity. The study highlights the key role of the S28P substitution and provides information that will aid the rational design of soluble variants of IAPP. The variants studied here offer a system to further explore features which control IAPP toxicity.