Uncoupling of the hnRNP Npl3p from mRNAs during the stress-induced block in mRNA export

Npl3p, the major mRNA binding protein of the yeast Saccharomyces cerevisiae shuttles between the nucleus and the cytoplasm. A single amino acid change in the carboxyl terminus of Npl3p (E409 --> K) renders the mutant protein largely cytoplasmic because of a delay in its import into the nucleus. This import defect can be reversed by increasing the intracellular concentratio n of Mtr10p, the nuclear import receptor for Npl3p. Conversely, using this mutant, we show that Npl3p and mRNA export out of the nucleus is significan tly slowed in cells bearing mutations in XPO1/CRM1, which encodes the expor t receptor for NES-containing proteins and in RAT7, which encodes an essent ial nucleoporin. Interestingly, following induction of stress by heat shock , high salt, or ethanol, conditions under which most mRNA export is blocked , Npl3p is still exported from the nucleus. The stress-induced export of Np l3p is independent of both the activity of Xpo1p and the continued selectiv e export of heat-shock mRNAs that occurs following stress. UV-cross-linking experiments show that Npl3p is bound to mRNA under normal conditions, but is no longer RNA associated in stressed cells. Taken together, we suggest t hat;the uncoupling of Npl3p and possibly other mRNA-binding proteins from m RNAs in the nucleus provides a general switch that regulates mRNA export. B y this model, under normal conditions Npl3p is a major component of an expo rt-competent RNP complex. However, under conditions of stress, Npl3p no lon ger associates with the export complex, rendering it export incompetent and thus nuclear.