SP1 IS PHOSPHORYLATED AND ITS DNA-BINDING ACTIVITY DOWN-REGULATED UPON TERMINAL DIFFERENTIATION OF THE LIVER

Using nuclear extracts prepared hom rat liver it was demonstrated that binding of a transcription factor to site II of the D-site binding pr otein promoter could be induced by dephosphorylation of these extracts . Competition band shifts and supershift assays reveal this protein to be the general transcription factor Sp1. Phosphorylation of Sp1 appea rs to occur as a result of terminal differentiation of the liver. Prot eins from both 1-day-old rat liver and adult liver undergoing regenera tion have less of the phosphorylated form of Sp1 present with conseque nt increased DNA binding activity. Sp1 is similarly phosphorylated in brain, kidney, and spleen with phosphatase treatment of the extracts s ignificantly increasing the level of DNA binding activity. Dephosphory lation of Sp1 results in a 10-fold increase in the affinity of Sp1 for its cognate site. Two-dimensional gel electrophoresis reveals that ap proximately 20% of the detectable protein appears to be in the phospho rylated form in adult liver extracts. Another protein with similar cha racteristics also appears to be present in the liver, Decreasing Sp1 D NA binding activity by phosphorylation may be an important mechanism f or regulating gene expression, and possibly bringing about growth arre st during terminal differentiation.