Question

Suppose that a single protein is significantly overexpressed in one of two different cell lines. The proteins are identical, but Northern blot analysis indicates that the mRNA for the protein in the overexpressing cell line is about 200 base pairs shorter. Subsequent reverse transcriptase PCR has revealed that the truncation occurs in the 3ʹ untranslated region. How might this lead to overexpression of the protein?

Answer 1

Explanation:

Many mRNAs have sequences of 50 - 150 nt rich in A and U in the 3' untranslated region. These sequences, called AU-rich elements, are important to regulate mRNA stability, because they bind proteins that target the mRNA for degradation.

When a protein binds to the AU-rich element of the mRNA, the polyA tail can be shortened and/or the 5'cap is removed, promoting mRNA degradation. In addition, the AU-rich elements in some mRNAs have been shown to inhibit protein translation.

The mRNA of the overexpressing cell line is about 200nt shorter than the normal cell line; this could mean that it has a deletion in the 3'UTR comprising an AU-rich element. If this is missing, the mRNA will have an extended half-life (because it won't be targeted for degradation) or it will be translated more. As a result, the protein expression will be increased.