Synapsin I is expressed in epithelial cells: localization to a unique trans-Golgi compartment

Synapsin I is abundant in neural tissues. Its phosphorylation is thought to regulate synaptic vesicle exocytosis in the pre-synaptic terminal by media ting vesicle tethering to the cytoskeleton. Using anti-synapsin antibodies, we detected an 85 kDa protein in liver cells and identified it as synapsin I. Like brain synapsin I, nonneuronal synapsin I is phosphorylated in vitr o by protein kinase A and yields identical P-32-peptide maps after limited proteolysis. We also detected synapsin I mRNA in liver by northern blot ana lysis. These results indicate that the expression of synapsin I is more wid espread than previously thought. Immunofluorescence analysis of several non -neuronal cell lines localizes synapsin I to a vesicular compartment adjace nt to trans-elements of the Golgi complex, which is also labeled with antib odies against myosin II; no sub-plasma membrane synapsin I is evident. We c onclude that synapsin I is present in epithelial cells and is associated wi th a trans-Golgi network-derived compartment; this localization suggests th at it plays a role in modulating post-TGN trafficking pathways.