A Novel Synaptic Vesicle‐Associated Phosphoprotein: SVAPP‐120

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Abstract
Generation of antibodies and direct protein sequencing were used to identify and characterize proteins associated with highly purified synaptic vesicles from rat brain. A protein doublet of low abundance of 119 and 124 kDa apparent molecular mass [synaptic vesicle‐associated phosphoprotein with a molecular mass of 120 kDa (SVAPP‐120)] was identified using polyclonal antibodies. SVAPP‐120 was found to copurify with synaptic vesicles and to be enriched in the purified synaptic vesicle fraction to the same extent as synapsin I. Like synapsin I, SVAPP‐120 is not an integral membrane protein because it was released from synaptic vesicles by high salt concentrations. This protein was demonstrated to be brain specific, and its distribution in various brain regions paralleled the distribution of synapsin I and synaptophysin. During the postnatal development of the rat cortex and cerebellum, its expression correlated with synaptogenesis. SVAPP‐120 was demonstrated to be a phosphoprotein both in vivo and in vitro. It was shown to be phosphorylated on serine and to a lesser extent on threonine residues. These results provide evidence that SVAPP‐120 represents a novel synaptic vesicle‐associated phosphoprotein. In addition, aldolase, a glycolytic enzyme, and αc‐adaptin, a clathrin assembly‐promoting protein, were identified on purified synaptic vesicles by direct protein sequencing.