Mechanism of Strand-Specific Smooth Muscle α-Actin Enhancer Interaction by Purine-Rich Element Binding Protein B (Purβ)

2009-07-14T00:00:00Z (GMT) by Jon E. Ramsey Robert J. Kelm
Expression of the smooth muscle α-actin gene in growth-activated vascular smooth muscle cells and stromal fibroblasts is negatively regulated by members of the Pur family of single-stranded DNA/RNA-binding proteins. In particular, Purα and Purβ are postulated to repress transcription by forming helix-destabilizing complexes with the sense strand of an asymmetric polypurine−polypyrimidine tract containing a canonical MCAT enhancer motif in the 5′ region of the gene. Herein, we establish the mechanism of Purβ binding to the purine-rich strand of the enhancer using quantitative methods and purified components. Initial evaluation of DNA-binding specificity and equilibrium stoichiometry via colorimetric-, autoradiographic-, and fluorescence-based assays suggested that Purβ interacts with two distinct G/A-rich sites within the nominal single-stranded enhancer element to form a high-affinity 2:1 protein:DNA complex. Statistical mechanical analyses of band shift titrations of the nominal element in conjunction with DNase I footprint titrations of the extended smooth muscle α-actin 5′-flanking region demonstrated that assembly of the nucleoprotein complex likely occurs in a sequential, cooperative, and monomer-dependent fashion. Resolution of the microscopic energetics of the system indicated that monomer association with two nonidentical sites flanking the core MCAT motif accounts for the majority of the intrinsic binding affinity of Purβ with intersite cooperativity contributing an ∼12-fold increase to the stability of the nucleoprotein complex. These findings offer new insights into the mechanism, energetics, and sequence determinants of Purβ repressor binding to a biologically relevant, contractile phenotype-regulating cis-element while also revealing the thermodynamic confines of putative Purβ-mediated effects on DNA structure.