Characterization of novel Pur alpha-binding proteins in mouse brain

Pur alpha is an abundant protein in the brain and binds to a (GGN)n sequence, PUR element. It has been shown that Pur alpha not only interacts with single stranded DNA and RNA, but also with various proteins. In the present study, we tried to search for Pur alpha-binding proteins (PurBPs) in mouse brain by the overlay assay with GST-Pur alpha as a ligand. Three PurBPs of 35, 38 and 40 kDa were found mostly in the nuclear extract (N.Ext.) and they were not detected by the pretreatment of N.Ext. with trypsin, but not with RNase or DNase. The three PurBPs disappeared by the addition of ssCRE (single stranded cAMP response element) containing a PUR element, but not by DeltaGGN ssCRE (deletion of the PUR element from the ssCRE). The PurBPs were abundantly expressed in the brain as Pur alpha. We also determined a region in Pur alpha which is required for the association with the PurBPs by using deletion mutants of Pur alpha. These biochemical properties of the PurBPs are different from the reported nuclear Pur alpha-binding proteins such as Sp1 and pRb.