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J. Biol. Chem., Vol. 279, Issue 44, 46286-46294, October 29, 2004

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ASH1 mRNA Anchoring Requires Reorganization of the Myo4p-She3p-She2p Transport Complex^*

Graydon B. Gonsalvez, Jaime L. Little, and Roy M. Long¶

From the Department of Microbiology and Molecular Genetics, Medical College of Wisconsin, Milwaukee, Wisconsin 53226

One mechanism by which cells post-transcriptionally regulate gene expression is via intercellular and intracellular sorting of mRNA. In Saccharomyces cerevisiae, the localization of ASH1 mRNA to the distal tip of budding cells results in the asymmetric sorting of Ash1p to daughter cell nuclei. Efficient localization of ASH1 mRNA depends upon the activity of four cis-acting localization elements and also upon the activity of trans-factors She2p, She3p, and Myo4p. She2p, She3p, and Myo4p have been proposed to form an ASH1 mRNA localization particle. She2p directly and specifically binds each of the four ASH1 cis-acting localization elements, whereas She3p has been hypothesized to function as an adaptor by recruiting the She2p-mRNA complex to Myo4p, a type V myosin. The Myo4p-She3p-She2p heterotrimeric protein complex has been proposed to localize mRNA to daughter cells using polarized actin cables. Here we demonstrate that whereas the predicted Myo4p-She3p-She2p heterotrimeric complex forms in vivo, it represents a relatively minor species compared with the Myo4p-She3p complex. Furthermore, contrary to a prediction of the heterotrimeric complex model for ASH1 mRNA localization, ASH1 mRNA artificially tethered to She2p is not localized. Upon closer examination, we found that mRNA tightly associated with She2p is transported to daughter cells but is not properly anchored at the bud tip. These results are consistent with a model whereby anchoring of ASH1 mRNA requires molecular remodeling of the Myo4p-She3p-She2p heterotrimeric complex, a process that is apparently altered when mRNA is artificially tethered to She2p.

Received for publication, June 1, 2004 , and in revised form, August 16, 2004.

^* This work was supported by National Institutes of Health Grant GM60392 and the Pew Scholars Program in the Biomedical Sciences. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Present address: Dept. of Genetics, Case Western Reserve University, BRB 739A, 10900 Euclid Ave., Cleveland, OH 44106-4955.

^¶ To whom correspondence should be addressed: Dept. of Microbiology and Molecular Genetics, Medical College of Wisconsin, 8701 Watertown Plank Rd., Milwaukee, WI 53226. Tel.: 414-456-8423; Fax: 414-456-6535; E-mail: rlong{at}mcw.edu.

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