Lamination and spherulite-like compaction of a hormone’s native amyloid-like nanofibrils: spectroscopic insights into key interactions

The native hierarchical self-assembly process of natural somatostatin-14, a highly aromatic and charged peptide hormone involved in various inhibitory functions, was investigated mainly using vibrational spectroscopy (ATR-FTIR and Raman scattering) combined with electron microscopy. Generic kinetic features of amyloid fibrillogenesis were confirmed for the somatostatin-14 case, together with new insights into key interactions involved in the further hierarchical assembly of the somatostatin-14 nanofibrils into i) laterally associated nanofibers and ii) spherulite-like amyloid droplets resulting from the compaction of the nanofibers. In particular, the key role of aromatic side-chains in both fibrillogenesis and the association of the nanofibrils into higher order structures could be followed. It is proposed that the compaction propensity of the somatostatin-14 nanofibrils is relevant to the current hypothesis of the biological function of hormone self-assembly processes: hormone storage inside secretory granules.