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

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.

Quality control of spliced mRNAs requires the shuttling SR proteins Gbp2 and Hrb1

“…Together with our previous finding showing that the export of Gbp2, Hrb1 and Npl3 is blocked in a mex67 mutant (ref. 17 and Fig. 1b), these data indicate that all SR proteins utilize Mex67 as an export receptor.…”

“…In an attempt to identify specific functions for the SR proteins, we screened for factors that inhibit the nuclear export of a Gbp2 variant (Gbp2 c -GFP), which is impaired in the speed of nuclear import and thus is equally distributed in both compartments at steady state 17 . We identified several late splicing factor mutants that all block the export of Gbp2 (Fig.…”

“…Only few of them accompany the mature mRNAs to the cytoplasm 16 . In yeast, three SR-like mRNAbinding proteins exist, termed Npl3, Gbp2 and Hrb1, which shuttle from the nucleus to the cytoplasm when bound to the mature mRNA molecules [17][18][19] . All three yeast SR proteins are recruited to the mRNAs co-transcriptionally, Npl3 early by RNA polymerase II and Gbp2 and Hrb1 via the THO complex, a tetrameric complex consisting of Hpr1, Tho2, Mft1 and Thp2, that associates with the elongating RNApolymerase and has been implicated in transcription elongation, mRNA export and genome stability 5,17,20,21 .…”

“…In yeast, three SR-like mRNAbinding proteins exist, termed Npl3, Gbp2 and Hrb1, which shuttle from the nucleus to the cytoplasm when bound to the mature mRNA molecules [17][18][19] . All three yeast SR proteins are recruited to the mRNAs co-transcriptionally, Npl3 early by RNA polymerase II and Gbp2 and Hrb1 via the THO complex, a tetrameric complex consisting of Hpr1, Tho2, Mft1 and Thp2, that associates with the elongating RNApolymerase and has been implicated in transcription elongation, mRNA export and genome stability 5,17,20,21 . Upon mRNA maturation, Npl3 and in higher eukaryotes the three shuttling SR proteins (SRSF1, SRSF3 and SRSF7) have been suggested to function as adapters between the mRNA and the export receptor Mex67 (TAP/NXF1 in higher eukaryotes) to promote mRNA export to the cytoplasm 16 .…”

Translation regulatory factor RBM3 is a proto-oncogene that prevents mitotic catastrophe

“…Nucleocytoplasmic shuttling of RBM3 is similar to that of hnRNP proteins that export mRNA from the nucleus (Pinol-Roma, 1997;Shyu and Wilkinson, 2000;Hacker and Krebber, 2004;Carpenter et al, 2006). Previously, Steitz and colleagues have speculated that HuR may bind to ARE-containing mRNAs and remain associated during transit through the nuclear envelope (Fan and Steitz, 1998b).…”