We have previously shown that heterogeneous nuclear ribonucleoprotein C (hnRNP C) and nucleolin bound specifically to a 29 nt sequence in the 3'-untranslated region of amyloid precursor protein (APP) mRNA. Upon activation of peripheral blood mononuclear cells, hnRNP C and nucleolin acquired APP mRNA binding activity, concurrent with APP mRNA stabilization. These data suggested that the regulated interaction of hnRNP C and nucleolin with APP mRNA controlled its stability. Here we have directly examined the role of the cis element and trans factors in the turnover and translation of APP mRNA in vitro. In a rabbit reticulocyte lysate (RRL) translation system, a mutant APP mRNA lacking the 29 nt element was 3-4-fold more stable and synthesized 2-4-fold more APP as wild-type APP mRNA. Therefore, the 29 nt element functioned as an APP mRNA destabilizer. RNA gel mobility shift assays with the RRL suggested the presence of endogenous nucleolin, but failed to show hnRNP C binding activity. However, wild-type APP mRNA was stabilized and coded for 6-fold more APP when translated in an RRL system supplemented with exogenous active hnRNP C. Control mRNAs lacking the 29 nt element were unaffected by hnRNP C supplementation. Therefore, occupancy of the 29 nt element by hnRNP C stabilized APP mRNA and enhanced its translation.
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