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Molecular and Cellular Biology, December 2004, p. 10479-10491, Vol. 24, No. 23
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.23.10479-10491.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Yeast Shuttling SR Proteins Npl3p, Gbp2p, and Hrb1p Are Part of the Translating mRNPs, and Npl3p Can Function as a Translational Repressor

Merle Windgassen,1,{dagger} Dorothée Sturm,1,{dagger} Iván J. Cajigas,2 Carlos I. González,2 Matthias Seedorf,3 Holger Bastians,1 and Heike Krebber1*

Institut für Molekularbiologie und Tumorforschung der Philipps-Universität Marburg, Marburg,1 Zentrum für Molekulare Biologie der Universität in Heidelberg, Heidelberg, Germany,3 Department of Biology, University of Puerto Rico—Río Piedras, San Juan, Puerto Rico2

Received 3 June 2004/ Returned for modification 8 July 2004/ Accepted 7 September 2004

A major challenge in current molecular biology is to understand how sequential steps in gene expression are coupled. Recently, much attention has been focused on the linkage of transcription, processing, and mRNA export. Here we describe the cytoplasmic rearrangement for shuttling mRNA binding proteins in Saccharomyces cerevisiae during translation. While the bulk of Hrp1p, Nab2p, or Mex67p is not associated with polysome containing mRNAs, significant amounts of the serine/arginine (SR)-type shuttling mRNA binding proteins Npl3p, Gbp2p, and Hrb1p remain associated with the mRNA-protein complex during translation. Interestingly, a prolonged association of Npl3p with polysome containing mRNAs results in translational defects, indicating that Npl3p can function as a negative translational regulator. Consistent with this idea, a mutation in NPL3 that slows down translation suppresses growth defects caused by the presence of translation inhibitors or a mutation in eIF5A. Moreover, using sucrose density gradient analysis, we provide evidence that the import receptor Mtr10p, but not the SR protein kinase Sky1p, is involved in the timely regulated release of Npl3p from polysome-associated mRNAs. Together, these data shed light onto the transformation of an exporting to a translating mRNP.

* Corresponding author. Mailing address: Institut für Molekularbiologie und Tumorforschung (IMT) der Philipps-Universität Marburg, Emil-Mannkopff-Str. 2, 35037 Marburg, Germany. Phone: 49 6421 286 6773. Fax: 49 6421 286 5932. E-mail: krebber{at}imt.uni-marburg.de.

{dagger} M.W. and D.S. contributed equally to this work.

Molecular and Cellular Biology, December 2004, p. 10479-10491, Vol. 24, No. 23
0022-538X/04/$08.00+0     DOI: 10.1128/MCB.24.23.10479-10491.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.



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