Association of the yeast RNA-binding protein She2p with the tubular endoplasmic reticulum depends on membrane curvature.

Localization of mRNAs contributes to the generation and maintenance of cellular asymmetry in a wide range of organisms. In Saccharomyces cerevisiae, the so-called locasome complex with its core components Myo4p, She2p, and She3p localizes more than 30 mRNAs to the yeast bud tip. A significant fraction of these mRNAs encodes membrane or secreted proteins. Their localization requires, besides the locasome, a functional segregation apparatus of the cortical endoplasmic reticulum (ER), including the machinery that is involved in the movement of ER tubules into the bud. Colocalization of RNA-containing particles with these tubules suggests a coordinated transport of localized mRNAs and the cortical ER to the bud. Association of localized mRNAs to the ER requires the presence of the locasome component She2p. Here we report that She2p is not only an RNA-binding protein but can specifically bind to ER-derived membranes in a membrane curvature-dependent manner in vitro. Although it does not contain any known curvature recognizing motifs, the protein shows a binding preference for liposomes with a diameter resembling that of yeast ER tubules. In addition, membrane binding depends on tetramerization of She2p. In an in vivo membrane-tethering assay, She2p can target a viral peptide GFP fusion protein to the cortical ER, indicating that a fraction of She2p associates with the ER in vivo. Combining RNA- and membrane-binding features makes She2p an ideal coordinator of ER tubule and mRNA cotransport.