Differential Export Requirements for Shuttling Serine/Arginine-type mRNA-binding Proteins

Sabine Häcker,Heike Krebber

doi:10.1074/jbc.c300522200

Abstract

Messenger RNAs are transported to the cytoplasm bound to several shuttling mRNA-binding proteins. Here, we present the characterization of Hrb1, a novel component of the transported ribonucleoprotein complex in Saccharomyces cerevisiae. The protein is similar to the other two serine/arginine (SR)-type proteins in yeast, Gbp2 and Npl3. Hrb1 is nuclear at steady state and its import is mediated by the karyopherin Mtr10. Hrb1 binds to poly(A)+ RNA in vivo and its binding is significantly increased in MTR10 mutants, suggesting a role for Mtr10 in dissociating Hrb1 from the mRNAs. Interestingly, by comparing the export requirements of all three SR proteins we find similarities but also striking differences. While the export of all three proteins is dependent on the export of mRNAs in general, as no transport is observed in mutants defective in transcription (rpb1-1) or mRNA export (mex67-5), we find specific requirements for components of the THO complex, involved in transcription elongation. While both Hrb1 and Gbp2 depend on Mft1 and Hpr1 for their nuclear export, Npl3 is exported independently of both proteins. These findings suggest that Hrb1 and Gbp2, but not Npl3, might be loaded onto the growing mRNA via the THO complex components Mtf1 and Hrp1.

Highlights

Messenger RNAs are transported to the cytoplasm bound to several shuttling mRNA-binding proteins
While the export of all three proteins is dependent on the export of mRNAs in general, as no transport is observed in mutants defective in transcription or mRNA export, we find specific requirements for components of the THO complex, involved in transcription elongation
While both Hrb1 and Gbp2 depend on Mft1 and Hpr1 for their nuclear export, Npl3 is exported independently of both proteins. These findings suggest that Hrb1 and Gbp2, but not Npl3, might be loaded onto the growing mRNA via the THO complex components Mtf1 and Hrp1

Summary

Accelerated Publication

While the export of all three proteins is dependent on the export of mRNAs in general, as no transport is observed in mutants defective in transcription (rpb1-1) or mRNA export (mex67-5), we find specific requirements for components of the THO complex, involved in transcription elongation While both Hrb and Gbp depend on Mft and Hpr for their nuclear export, Npl is exported independently of both proteins. The import requirements of all three proteins are very similar, we find striking differences in their export requirements While both Hrb and Gbp are dependent on Mft and Hpr, two components of the THO complex, the export of Npl is independent of both components, suggesting differences in the recruitment of the SR proteins to the mRNAs. The cellular separation of transcription and translation in eukaryotic cells, requires transport across the nuclear envelope from one compartment into the other. One of the key players is Mex (TAP or NXF1 in metazoans), which recognizes the mature mRNA and mediates the interaction between the messenger ribonucleoparticles

EXPERIMENTAL PROCEDURES

RESULTS AND DISCUSSION

Sabine Häcker and Heike Krebber