Role of calcium ion and anionic phospholipids on the ubiquitin aggregation process

Ubiquitin (Ub) is a small and highly conserved protein, which participates in numerous biological processes. In eukaryotic cells, the Ubiquitin-Proteasome system (UPS) is the main pathway for eliminating misfolded or damaged proteins. The failure of the UPS towards misfolded proteins can lead to the formation of toxic oligomeric aggregates and appear to be involved in some neurodegenerative diseases including Parkinson’s, Alzheimer’s, amyotrophic lateral sclerosis, and prion diseases. The link between protein misfolding and aggregation, UPS, and neurodegenerative disorders is supported by the observation that protein aggregates within affected cells often contain Ub. Ub stability seems to be affected by different extrinsic factors including metal ions and membrane lipids. Copper (Cu2+) and zinc (Zn2+) are able to bind to Ub promoting its aggregation as well as anionic lipids acting as molecular chaperones. This study evaluated the effects of calcium ions (Ca2+) and anionic phospholipids on the Ub aggregation process. The formation of Ub aggregates was assessed using dot blot and SDS-page assays and the protein's ability to permeabilize the membrane was monitored through relaxometric and single-channel current measurements. Calcium ions and anionic phospholipids seem to favor the formation of oligomeric aggregates which incorporate into the membrane and form conductive units, suggesting a possible mechanism of Ub toxicity.