Neuronal Ca2+ sensor (NCS) proteins, Neurocalcin $\delta$ (NCALD) and Hippocalcin (HPCA) control many neuronal functions via their differential Ca2+-dependent translocation from the cytosol to the plasma membrane. In spite of the fact that both proteins have highly similar amino acid (AA) sequences, they demonstrate a great difference in Ca2+ affinity and kinetic properties of translocation to the plasma membrane. It has been suggested that these differences between the proteins are due to Ca2+-dependent dimerization of Neurocalcin $\delta$, which is not observed for Hippocalcin.
To test this hypothesis, Neurocalcin $\delta$ mutant (SKA Mut) revealing no dimerization in vitro, was used. The mutant and wild type NCALD were tagged by fluorescent proteins and co-expressed paired wise in cultured rat hippocampal neurons. To induce Ca2+-dependent translocations of proteins, we applied depolarizations of different duration using a patch clamp technique.
Surprisingly, disruption of Ca2+-dependent dimerization of NCALD had only a minor influence on Ca2+-sensitivity without a significant effect on decay time of translocation. Ca2+-dependent NCALD insertion into the plasma membrane occurring before Ca2+-dependent dimerization may explain this result. According to estimations conducted using our experimental results, characteristic diffusion time for NCALD insertion into the plasma membrane is about 3-fold lower than characteristic diffusion time for interaction of two NCALD in the dendritic tree of hippocampal neurons. In summary, Ca2+-dependent dimerization of Neurocalcin $\delta$ in the cytosol does not play a significant role in determination of biophysical properties of this protein.