Synapsins as major neuronal Ca2+/S100A1-interacting proteins

The mammalian S100A1 protein can activate the invertebrate myosin-associated giant protein kinase twitchin in a Ca²⁺ dependent manner by more than 1000-fold in vitro; however, no mammalian S100-dependent protein kinases are known. In an attempt to identify novel mammalian Ca²⁺/S100A1-regulated protein kinases, brain extracts were subjected to combined Ca²⁺ dependent affinity chromatography with S100A1 and an ATP analogue. This resulted in the purification to near-homogeneity of the four major synapsin isoforms Ia, Ib, IIa and IIb. All four synapsins were specifically affinity-labelled with the ATP analogue 5'-p-fluorosulphonylbenzoyladenosine. S100A1 bound to immobilized synapsin IIa in BIAcore experiments in a Ca²⁺ dependent and Zn²⁺-enhanced manner with submicromolar affinity; this interaction could be competed for with synthetic peptides of the proposed S100A1-binding sites of synapsin. Double-labelling confocal immunofluorescence microscopy demonstrated that synapsins and S100A1 are both present in the soma and neurites of PC12 cells, indicating their potential to interact in neurons in vivo.