Cysteine string Protein (CSP) is a neuronal chaperone that maintains normal neurotransmitter exocytosis and is essential for preventing presynaptic neuroclegeneration. CSP is phosphorylated in vivo on a single residue, Ser10, and this phosphorylation regulates its cellular functions, although the molecular mechanisms involved are unclear. To identify novel phosphorylation-specific binding partners for CSP, we used a pull-down approach using synthetic peptides and recombinant proteins. A single protein band was observed to bind specifically to a Ser10-phosphorylated CSP peptide (residues 4-14) compared to a non-phosphorylated peptide. This band was identified as 14-3-3 protein of various isoforms using mass spectrometry and Western blotting. PKA phosphorylation of full-length CSP protein Stimulated 14-3-3 binding, and this was abolished in a Ser10-Ala mutant CSP, confirming the binding site as phospho-Ser10. As both CSP and 14-3-3 proteins are implicated in neurotransmitter exocytosis and neurodegeneration, this novel phosphorylation-dependent interaction may help maintain the functional integrity of the synapse. (C) 2008 Elsevier Inc. All rights reserved.