The cis-acting elements involved in endonucleolytic cleavage of the 3′ UTR of human IGF-II mRNAs bind a 50 kDa protein

Site-specific cleavage of human insulin-like growth factor II mRNAs requires two cis-acting elements, I and II, that are both located in the 3′ untranslated region and separated by almost 2 kb. These elements can interact and form a stable RNA-RNA stem structure. In this study we have initiated the investigation of transacting factors involved in the cleavage of IGF-II mRNAs. The products of the cleavage reaction accumulate in the cytoplasm, suggesting that cleavage occurs in this cellular compartment. By electrophoretic mobility shift assays, we have identified a cytoplasmic protein with an apparent molecular weight of 48-50 kDa, IGF-II cleavage unit binding protein (ICU-BP), that binds to the stem structure formed by interaction of parts of the cis-acting elements I and II. The binding is resistant to high K+ concentrations and is dependent on Mg2+. In addition, ICU-BP binding is dependent on the cell density and correlates inversely with the IGF-II mRNA levels. In vivo cross-linking data show that this protein is associated with IGF-II mRNAs in vivo.