Characterization of neurocalcin delta membrane binding by biophysical methods

Neurocalcin delta (NCALD) is a member of the neuronal calcium sensors protein family. In the retina, NCALD is expressed by ganglion and amacrine cells. NCALD is composed of 4 EF-hand motifs but only 3 of them may bind calcium. The binding of calcium induces a conformational change of the protein which leads to the extrusion of its N-terminal myristoyl group as well as some hydrophilic residues. This mechanism, named calcium-myristoyl switch, is presumably involved in its membrane binding. The parameters responsible for the interaction of NCALD with membranes are only partially known. The purpose of this study was thus to gather more information on the membrane binding behavior of NCALD using lipid monolayers, including the influence of the lipid composition, the calcium and the myristoyl group. NCALD was injected underneath different lipid monolayers and this model membrane allowed the determination of the binding parameters as maximum insertion pressure (MIP) and synergy. The values of MIP are larger when monolayers were composed of a saturated phospholipid with phosphoethanolamine polar head. This trend is confirmed by polarization modulation infrared reflection absorption spectroscopy measurements. Moreover, the observations by fluorescence microscopy show that NCALD preferentially interacts with phospholipids which are in the liquid-condensed physical state, as found in membrane microdomains. This observation could explain the changes of NCALD expression level in the brains of patients suffering from Alzheimer's disease because of the alteration of lipid composition in microdomains structures.