Figure 7.

Possible high-affinity RNA binding modes by STAR protein dimers. A. Model for STAR protein dimerization using the monomeric structure of SF-1 bound to RNA aligned with the three-dimensional structure of the GLD-1 Qua1 dimer interface. The dashed red line represents the potential orientation of a contiguous stretch of single-stranded RNA connecting the two hexamer binding sites. B. Qua1-mediated STAR dimers likely present identical RNA binding surfaces by both protomers. The STAR domains are represented by polygons in yellow, light blue and dark blue corresponding to Qua1, KH and Qua2, respectively. C. In the case of GLD-1 bound to TGE RNA, the asymmetric complex would have one protomer binding the canonical hexamer in the usual mode while the other would recognize the upstream element in a distinct manner. D. Symmetric hexamer mode in the case of a 2:1 binding stoichiometry, as with a 12-mer RNA containing a single hexamer. In this case, each monomer binds RNA, represented here by red arrows, in an identical fashion.