Identification of nuclear matrix and associated proteins that bind the haptoglobin gene cis-element

To identify the major nuclear matrix proteins that bind to the rat haptoglobin gene cis-element, we isolated a soluble nuclear matrix protein fraction and analysed it by gel retardation. Two major DNA-binding proteins exhibiting different types of protein-DNA interactions were detected: a DNA sequence-specific 32-kDa isoform of transcription factor C/EBP beta, and a nuclear matrix protein p55 that bound to the DNA nonspecifically. During increased transcription of the haptoglobin gene in the course of the acute-phase reaction, the DNA-binding affinities and concentrations of these proteins in the soluble nuclear matrix fraction were increased. These data lend further evidence that the nuclear matrix is an active support structure that localizes gene regulatory proteins and participates in transcriptional regulation.