Abstract
The single amino acid mutation G26R in human apolipoprotein A-I (apoA-I
IOWA
) leads to the formation of β-secondary structure rich amyloid fibrils
in vivo
. Here we show that full-length apoA-I
IOWA
has a decreased lipid binding capability, an increased amino terminal sensitivity to protease, and a propensity to form annular protofibrils visible by electron microscopy. The molecular basis for the conversion of apolipoprotein A-I to a pro-amyloidogenic form was examined by electron paramagnetic resonance spectroscopy. Our recent findings [Lagerstedt, J. O., Budamagunta, M. S., Oda, M. N., and Voss, J. C. (2007)
J Biol Chem
, 282, 9143–9149] indicate that Gly26 in native apo-protein separates a preceding β-strand structure (residues 20–25) from a downstream largely α-helical region. The current study demonstrates that the G26R variant promotes a structural transition of positions 27–56 to a mixture of coil and β-strand secondary structure. Microscopy and staining by amyloidophilic dyes suggest that this alteration extends throughout the protein within one week of incubation
in vitro
, leading to insoluble aggregates of distinct morphology. The severe consequences of the Iowa mutation likely arise from the combination of losing the contribution of the native Gly residue in terminating β-strand propagation and the promotion of β structure when an Arg is introduced adjacent to succeeding residue of identical charge and size, Arg27.