Characterization of Two Nuclear Proteins that Interact with Cytochrome P‐450 1A2 mRNA

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Abstract
Regulation of the expression of the cytochrome P‐450 1a2 gene (cyp1a2) occurs mainly at the transcriptional level, but the molecular events involved in the induction process are partly unknown. Some reports have proposed involvement of post‐transcriptional mechanisms [Adesnik, M. & Atchison, M. (1986)Crit. Rev. Biochem. 19, 247–305; Silver, G. & Krauter, K. S. (1990)Mol. Cell. Biol. 10, 6765–6768], Here we report the identification of two proteins in the nuclear fraction of mouse liver, with specific binding characteristics towards CYP1A2 mRNA. The proteins have apparent molecular masses of 37 kDa and 46 kDa and exhibit a high affinity for a poly(U) motif in the 3′ untranslated region of CYP1A2 mRNA. This motif seems to be important for their specific and apparently competitive binding to CYP1A2 mRNA. Treatment of mice with an inducer of CYP1A2, 3‐methylcholanthrene, increases the binding of the 46‐kDa protein and decreases the binding of the 37‐kDa protein to the mRNA, suggesting that changes in the binding of the proteins to the mRNA could play a role in the upregulation of CYP1A2 mRNA by 3‐methylcholanthrene. Phosphorylation of the 46‐kDa protein, or of an intermediary factor, may play a role in its binding activity. Furthermore, the 46‐kDa but not the 37‐kDa protein is recognized by a monoclonal antibody against the heterogeneous nuclear ribonucleoprotein C, a nuclear protein probably involved in pre‐mRNA processing. While more work is needed to understand the function of the proteins that bind to the 3′ untranslated region of CYP1A2, it is possible that the 37‐kDa protein has a role in the maintenance of uninduced levels of CYP1A2 mRNA, while the 46‐kDa protein could be important in the maturation of elevated levels of CYP1A2 pre‐mRNA, during induction.