Amyloid fibrillation of an intrinsically disordered plant phloem protein AtPP16-1 under acidic condition.

Arabidopsis thaliana Phloem Protein 16-1 (AtPP16-1) is a 156-residue intrinsically disordered nucleic acid binding protein which is putatively involved in long-distance systemic transport of RNA to budding regions in plants. Dimerization or oligomerization of the protein at pH higher than about 4.1 leaves no apolar surface exposed for interaction with the dye 8-Anilinonaphthalene-1-sulfonate (ANS). The most stable monomeric state is found near pH 4 where the structure of the protein is determined to have three short β-strands and a single α-helix. By surveying the pH-dependent propensity of fibrillation we find the protein enters the amyloidogenic state at pH 2, 60 °C. The reaction product is not amorphous aggregate, but simple amyloid fibrils with sparse or no branching. The mean diameters of the fibril population scaled from AFM images are 13.2 and 21.2 nm for precursor aggregates (PA) and proto- or elongated fibrils, respectively. These values are somewhat larger than the fibril diameters generally cited, and the reason could be larger lateral association for both PA and protofibrils. The protein AtPP16-1 is strictly pH-selective in terms of its structure and stability, and the solution structure is known at pH 4. Under the conditions of pH 2 used here for fibrillation, the protein retains substantial secondary structure. Even if the pH and temperature conditions used for fibrillation are hardly physiological, there is a finite possibility that some aggregation of AtPP16-1 would occur in vivo, as the case of transglutaminase aggregates in the chloroplast of transplastomic plants, for example. The pH related problem has been discussed in detail, but the questions emanating are: do phloem proteins fibrillate in vivo, and if so what implication fibrillation has for plant physiology?