Title : Glycosylation sites flank phosphorylation sites on synapsin I : O-linked N-acetylglucosamine residues are localized within domains mediating synapsin I interactions
Abstract :
Synapsin I is concentrated in nerve terminals, where it appears to anchor synaptic vesicles to the cytoskeleton and thereby ensures a steady supply of fusion-competent synaptic vesicles
Although phosphorylation-dependent binding of synapsin I to cytoskeletal elements and synaptic vesicles is well characterized, little is known about synapsin I 's O-linked N-acetylglucosamine (O-GlcNAc) modifications
Here, we identified seven in vivo O-GlcNAcylation sites on synapsin I by analysis of HPLC-purified digests of rat brain synapsin I
The seven O-GlcNAcylation sites ( Ser55, Thr56, Thr87, Ser516, Thr524, Thr562, and Ser576 ) in synapsin I are clustered around its five phosphorylation sites in domains B and D
The proximity of phosphorylation sites to O-GlcNAcylation sites in the regulatory domains of synapsin I suggests that O-GlcNAcylation may modulate phosphorylation and indirectly affect synapsin I interactions
With use of synthetic peptides , however, the presence of an O-GlcNAc at sites Thr562 and Ser576 resulted in only a 66% increase in the Km of calcium/calmodulin-dependent protein kinase II phosphorylation of site Ser566 with no effect on its Vmax
We conclude that O-GlcNAcylation likely plays a more direct role in synapsin I interactions than simply modulating the protein's phosphorylation