Inhibition of Translation by mRNA Encoding NIPP‐1, a Nuclear Inhibitor of Protein Phosphatase‐1

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Abstract
Transient transfection of COS-1 cells with an expression vector for NIPP-1, a nuclear subunit of protein phosphatase-1, did not result in an overexpression of NIPP-1 protein, although the levels of mRNA encoding NIPP-1 increased dramatically. Moreover, high concentrations of NIPP-1 mRNA inhibited the translation in reticulocyte lysates of various unrelated mRNAs. This inhibition of translation was caused by the NIPP-1 messenger and not by the translation product, since mutation of the start codon abolished NIPP-1 protein production, but had no influence on the translational inhibition. Analysis of deletion mutants showed that the inhibition was mediated by a 0.5-kb fragment in the 5′-end of the NIPP-1 mRNA. This region, when inserted in the 5′-untranslated region of the β-galactosidase messenger, inhibited the translation of β-galactosidase mRNA in COS-1 cells. A predicted highly stable secondary structure G= -239.5 kJ/mol) is present between residues 300 and 500 of NIPP-1 mRNA. The possible importance of this structure in the translational inhibition is discussed.