Purification and Partial Amino Acid Sequence of a Fetal Brain-Specific 33-kDa Protein

We have analyzed the qualitative and quantitative changes in protein expression in rat brains during aging. To detect proteins transiently expressed in prenatal brain, protein maps of brain cytosol from different stages were compared. We found that more than 30 proteins are expressed specifically in embryonic brain. Among these proteins is a 33-kDa protein, p33, which was predominantly expressed in the prenatal stages and reduced in the early postnatal stage, was purified and partially characterized. Partial amino acid sequences of tryptic fragments of p33 showed that this protein had a sequence highly homologous with that of PCNA/cyclin. In addition, the p33 molecule also had unidentified sequences that showed no homology with any proteins on computer homology search. Western blot analysis using anti-phosphoamino acid antibodies showed that p33 reacted only with anti-phosphothreonine antibody. These results indicate that p33 is a novel fetal brain-specific protein having a PCNA/cyclin homologous sequence. We also believe that p33 might be a molecular species or a member of the PCNA/cyclin family involved in cell proliferation or differentiation or both of the fetal central nervous sytem on the basis of homology and phosphorylation of threonine residues.