Native functions of amyloid

Abstract

Amyloids are highly ordered protein/peptide aggregates with cross-β-sheet rich structure. Amyloids are originally associated with many neurodegenerative diseases including Alzheimer's, Parkinson's and Type II diabetes. The natively structured or unstructured proteins adopt partially folded conformation and subsequently selfassociates through nucleation dependent polymerization to form amyloid fibrils. These fibrils are very stable, resistant to proteases and to harsh environmental conditions. Recently, several studies have indicated that amyloid fibrils are also abundant in living organisms from prokaryotes to eukaryotes, where amyloids are evolved to perform native functions of the host. Such amyloids are termed as 'functional amyloids'. Curli in E. coli and Het-s in Podospora anserina are well known examples of functional amyloids in bacteria and fungi respectively. Yeast prions do not cause cell death rather help the host to survive in certain environmental conditions. In mammals, Pmel17 forms amyloid inside the melanosome, where it is involved in skin pigmentation. Moreover, recent studies have suggested that peptide/protein hormones in pituitary secretory granules are stored in amyloid-like aggregates. In this chapter, we summarize the recent discoveries of functional amyloids, where amyloid fibrils are evolved for an organism's survival rather than creating only diseases.