NH₂ terminus of PTB-associated splicing factor binds to the porcine P450scc IGF-I response element

An insulin-like growth factor (IGF) I response element (IGFRE) in the porcine P-450 cholesterol side-chain cleavage gene (P450scc) regulates transcription through the binding of two proteins, Sp1 and polypyrimidine tract-binding protein-associated splicing factor (PSF). PSF is a component of spliceosomes and contains RNA-binding domains. In this study, we localized the NH₂-terminal amino acid residues necessary for binding of PSF to the IGFRE. Three COOH-terminal truncated proteins (aa 304, 214, and 134) of PSF were designed to empirically partition the NH₂-terminal region while excluding the RNA-binding domains. Southwestern analysis showed that only the largest expressed truncated protein, P3, strongly bound the porcine P450scc IGFRE. Truncated PSF protein expression in Y1 adrenal cells showed that P3 repressed transcriptional activity of the IGFRE similar to full-length PSF, whereas P2 (minimal binding to the IGFRE) had no effect. In conclusion, the NH₂-terminal region of PSF contains the amino acid residues necessary for binding to the porcine P450scc IGFRE and repressing the transcriptional activity of the element.