L-arginine interferes with functional studies of amyloid proteins

Intrinsically disordered proteins/regions are abundant in cancer signalling pathways and neurodegenerative diseases like Parkinson's disease, amyotrophic lateral sclerosis, Alzheimer's disease, etc. Purification of intrinsically disordered proteins can be challenging due to their sticky nature. For intrinsically disordered amyloid proteins, in-vitro aggregation studies are ideal experiments to study their liquid to solid transition. However, over-expression of these proteins in E. coli often results in insoluble protein fraction that ends up in cell-pellet as inclusion bodies, on lysis and centrifugation. Supplementing purification buffers with l-arginine is known to increase the solubility of proteins. For most of the structured proteins increasing solubility translates into a higher yield of functional proteins. However, for aggregation prone proteins associated with neurodegenerative diseases, like α-synuclein (Parkinson's disease), Aβ (Alzheimer's disease), fused in sarcoma (amyotrophic lateral sclerosis), etc. inclusion of l-arginine might interfere with aggregation studies. To test our hypothesis, we purified aggregation prone α-synuclein and fused in sarcoma protein in the presence and absence of l-arginine and studied their fibrillization. While recombinant FUS is difficult to prepare, purification of α-synuclein is well established but in all the protocols a significant amount of protein remains as insoluble fraction in the pellet. Inclusion of l-arginine increases the yield of protein purification by about 3 folds for both the proteins, but the resulting protein does not aggregate into fibrils thus showing that increased solubility of amyloid proteins (α-synuclein and fused in sarcoma) in the presence of l-arginine is not suitable for aggregation studies.