Proteomic analysis of nuclear factors binding to an intronic enhancer in the myelin proteolipid protein gene

The myelin proteolipid protein gene (Plp1) encodes the most abundant protein found in CNS myelin, accounting for nearly one-half of the total protein. Its expression in oligodendrocytes is developmentally regulated - peaking during the active myelination period of CNS development. Previously, we have identified a novel enhancer (designated ASE) in intron 1 DNA that appears to be important in mediating the surge of Plp1 gene activity during the active myelination period. Evidence suggests that the ASE participates in the formation of a specialized multi-protein/DNA complex called an enhanceosome. The current study describes an optimized, five-step, DNA affinity chromatography purification procedure to purify nuclear proteins from mouse brain that bind to the 85-bp ASE sequence, specifically. Electrophoretic mobility shift assay analysis demonstrated that specific DNA-binding activity was retained throughout the purification procedure, resulting in concomitant enrichment of nucleoprotein complexes. Identification of the purported regulatory factors was achieved through mass spectrometry analysis and included over 20 sequence-specific DNA-binding proteins. Supplementary western blot analyses to determine which of these sequence-specific factors are present in oligodendrocytes, and their developmental and regional expression in whole brain, suggest that Pur alpha and Pur beta rank highest among the candidate factors as constituents of the multi-protein complex formed on the ASE.