We previously reported a new brain-specific protein with a molecular mass of 14 kDa, specifically present in synapses around neurons but not in glial cells [Nakajo, S., Omata, K., Aiuchi, T., Shibayama, T., Okahashi, I., Ochiai, H., Nakai, Y., Nakaya, K. & Nakamura, Y. (1990) J. Neurochem. 55, 2031-2038]. In the present study, we determined the primary structure of this protein, found that it is phosphorylated in vitro and in vivo, and designated it phosphoneuroprotein 14 (PNP 14). The protein is a single polypeptide with 134 amino acid residues (molecular mass = 14122 Da), and it contains a hydrophobic region at the center of the molecule. The carboxy-terminal region has all seven proline residues, and is rich in glutamic acid, which contribute to the acidic property of the protein. The amino-terminal region possesses four unique repetitive motifs, Glu(Ser)-Lys-Thr-Lys-Glu(Gln)-Gly(Gln)-Val(Ala). When a cytosolic fraction prepared from rat cerebral cortex was incubated with [gamma-32P]ATP, 32P was incorporated into PNP 14. Phosphorylated PNP 14 was immunoprecipitated from rat brain synaptosomes labeled metabolically with [32P]orthophosphate. Injection of [32P]orthophosphate into the third ventricle of rat brain resulted in incorporation of radioactive phosphate into PNP 14. We have also found that Ca2+, calmodulin-dependent protein kinase II phosphorylates serine residue(s) of PNP 14 in vitro. The results suggest that PNP 14 may be important to neuronal cells, but not to glial cells, and that its physiological functions may be controlled by the phosphorylation reaction.