Insight into the Interaction between Neuronal Calcium Sensors and Insulin

Calcium is crucial in insulin biology and Ca ²⁺ sensor proteins enforced in insulin release and signalling. The neuronal calcium sensor proteins (NCS) such as NCS-1 and VILIP are shown to be involved in insulin secretion from β-pancreatic cells. However, the expression of different NCS proteins in the pancreas and their functional significance and role in pathologies remained unexplored. The present work, through different biophysical methods, presented that NCS proteins interact with insulin. NCS-1, the founder member of NCS family proteins interacts with insulin in a Ca ²⁺ independent manner and Ca ²⁺ enhances the affinity of the interaction. The evolutionarily conserved cryptic EF-hand in NCS proteins was found to be an essential commodity for binding insulin. The presence of Ca ²⁺ binding first EF-hand abolishes the interaction with insulin and suggests the significance of non-functional EF-hand. The fluorescence and circular dichroism (CD) spectroscopy show that insulin interaction induces structural changes in NCS-1, which is demonstrated by size exclusion chromatography and analytical ultra-centrifugation. The autism mutant NCS-1-R102Q relatively retained insulin binding properties but with a significant difference in binding thermodynamics. Considering substantial sequence similarity among different NCS proteins and localisation in the pancreas, we examined the insulin interaction with the neurocalcin delta (NCALD). The NCALD shows metal ion-independent insulin binding and contrary to NCS-1, the Ca ²⁺ abolishes the insulin binding. This highlights the differential regulation of Ca ²⁺ towards insulin interaction in NCS protein. Conclusively the present work highlight that NCS proteins interact with insulin and further investigation would aid to understand the significance of NCS proteins in insulin physiology/pathophysiology and possible new molecular targets in diabetes.