Coordination of Endoplasmic Reticulum and mRNA Localization to the Yeast Bud

Localization of messenger RNAs and local protein synthesis contribute to asymmetric protein distribution not only of cytoplasmic but also of membrane or secreted proteins [1, 2]. Since synthesis of the latter protein classes occurs at the rough endoplasmic reticulum (ER), mRNA localization and distribution of ER should be coordinated. However, this coordination is not yet understood. In yeast, mRNA localization to the growing bud depends on the myosin Myo4p, its adaptor She3p, and the specific RNA binding protein She2p [3]. These proteins mediate the localization of 23 mRNAs including ASH1 mRNA and mRNAs encoding membrane proteins [4]. In addition, Myo4p and She3p are required for segregation of cortical ER to the bud [5]. Here we show, with ASH1 mRNA as a model mRNA, that localizing messenger ribonucleoprotein (mRNP) particles comigrate with tubular ER structures to the bud, which requires the RNA binding protein She2p. Coordinated movement of the ASH1 mRNP with ER tubules but not their association with each other depends on Myo4p and She3p. Subcellular fractionation experiments demonstrate a cosegregation of ER and She2p, which is independent of Myo4p, She3p, or polysomes. Our findings suggest a novel model for mRNA localization that involves association of She2p and mRNPs with ER tubules and myosin-dependent cotransport of tubules and localized mRNPs.