Biophysical Evidence for the Amyloid Formation of a Recombinant Rab2 Isoform of Leishmania donovani.

BACKGROUND The most fatal form of Visceral leishmaniasis or kala-azar is caused by the intracellular protozoan parasite Leishmania donovani. The life cycle and the infection pathway of the parasite are regulated by the small GTPase family of Rab proteins. The involvement of Rab proteins in neurodegenerative amyloidosis is implicated in protein misfolding, secretion abnormalities and dysregulation. The inter and intra-cellular shuttlings of Rab proteins are proposed to be aggregation-prone. However, the biophysical unfolding and aggregation of protozoan Rab proteins is limited. Understanding the aggregation mechanisms of Rab protein will determine their physical impact on the disease pathogenesis and individual health. OBJECTIVE This work investigates the acidic pH-induced unfolding and aggregation of a recombinant Rab2 protein from L. donovani (rLdRab2) using multi-spectroscopic probes. METHODS The acidic unfolding of rLdRab2 induced at acidic pH is characterised by intrinsic fluorescence and ANS assay, while aggregation is determined by Thioflavin-T and 90⁰ light scattering assay. Circular dichroism determined the secondary structure of monomers and aggregates. The aggregate morphology was imaged by transmission electron microscopy. RESULTS rLdRab2 was modelled to be a Rab2 isoform with loose globular packing. The acidinduced unfolding of the protein is a plausible non-two-state process. At pH 2.0, a partially folded intermediate (PFI) state characterised by ~ 30 % structural loss and exposed hydrophobic core was found to accumulate. The PFI state slowly converted into well-developed protofibrils at high protein concentrations demonstrating its amyloidogenic nature. The native state of the protein was also observed to be aggregation-prone at high protein concentrations. However, it formed amorphous aggregation instead of fibrils. CONCLUSION To our knowledge, this is the first study to report in vitro amyloid-like behaviour of Rab proteins in L donovani. This study provides a novel opportunity to understand the complete biophysical characteristics of Rab2 protein of the lower eukaryote, L. donovani.