Model of stepwise SHE complex assembly. (A) One She2p tetramer (red) binds two zip code RNAs (black). In the presence of transcripts with two or more zip code elements, higher-order assembly (as shown in D) might occur already at this stage. (B) One Myo4p monomer (blue) forms a constitutive co-complex with one She3p dimer (orange). The N-terminal coiled-coil mediates dimerization of She3p as well as interaction with the Myo4p C terminus. This single-headed motor complex is unable to promote processive transport (Dunn et al., 2007; Hodges et al., 2008). (C) The RNA:She2p subcomplex assembles with two Myo4p:She3p subcomplexes forming the mature SHE complex. The C terminus of She3p binds directly to zip code RNA and to helix E of She2p (Müller et al., 2011). This interaction ensures highly specific RNA recognition and incorporates two Myo4p copies into one mRNP. She2p-mediated dimerization of Myo4p enables processive transport of the SHE complex. (D) Transcripts with more than one zip code element induce multimerization of SHE complexes. Because the She2p tetramer has two RNA binding sites, several RNAs are incorporated, and large mRNPs of variable size are formed. Such higher-order mRNPs contain multiple Myo4p dimers that might enhance sustained transport in vivo (Chartrand et al., 2002; Chung and Takizawa, 2010). (E) Calculation of SHE particle properties in cells based on our model, the minimal/mean/maximal number of ASH1 mRNAs observed per transport particle in vivo (marked by an asterisk; Haim et al., 2007), and the reported 88 ASH1 mRNA copies per cell (Trcek et al., 2012). Details on the calculation are given in the Materials and methods section and in Tables S1 and S2.