An elevated level of the mRNA exporter Mex67-Mtr2 in nuclear mRNPs impairs nuclear mRNA export.

In eukaryotes, nuclear messenger RNA (mRNA) export is a crucial step in gene expression, mediated by the conserved mRNA exporter Mex67-Mtr2 in Saccharomyces cerevisiae and NXF1-NXT1 in humans. Mex67-Mtr2 is recruited to the mRNA by the adaptors Hpr1, Nab2, Yra1, and Npl3, which play important yet incompletely understood roles in this process. Here, we uncover that, counterintuitively, an excess of Mex67 in nuclear messenger ribonucleoprotein particles (mRNPs) impairs nuclear mRNA export. Cells lacking Hpr1, which exhibit a nuclear mRNA export defect, show elevated levels of Nab2, Yra1, and Mex67 in nuclear mRNPs. Remarkably, overexpression of either Nab2 or Yra1 in Δhpr1 cells suppresses this export defect and simultaneously decreases the Mex67 level in nuclear mRNPs to those of wild-type cells. Importantly, a nuclear mRNA export defect is not inherently associated with an elevated Mex67 level in nuclear mRNPs, indicating that the increased Mex67 level in nuclear mRNPs of Δhpr1 cells is likely the cause rather than the consequence of the nuclear mRNA export defect. Thus, the precise regulation of the Mex67-Mtr2 level in nuclear mRNPs is essential for efficient nuclear mRNA export.