Pur beta is a single-stranded nucleic acid-binding protein implicated in the injury-induced repression of genes encoding certain muscle-restricted isoforms of actin and myosin expressed in the heart, skeletal muscle, and vasculature To better understand how the modular arrangement of the primary sequence of Pur beta affects the higher order structure and function of the protein, purified recombinant Pun beta was subjected to partial proteolysis in an attempt to identify a well-folded truncation protein that retained purine-rich single-stranded DNA-binding activity Limited tryptic digestion of Pur beta liberated a core similar to 30 kDa fragment corresponding to residues 29-305 as determined by epitope mapping and mass spectrometry. Size exclusion chromatography indicated that the isolated core fragment retains the ability to self-associate while circular dichroism analysis confirmed that the Pur beta core domain is stably folded in the absence of glycine-rich N- and C-terminal sequences Comparative DNA-binding assays revealed that the isolated core domain interacts with purine-rich cis-elements from the smooth muscle alpha-actin gene with similar specificity but increased affinity compared to full-length Pun beta These findings suggest that the highly conserved modular repeats of Pur beta fold to form a core functional domain, which mediates the specific and high affinity binding of the protein to single-stranded DNA (C) 2010 Elsevier Inc All rights reserved