Figure 3.

The Tripartite Architecture of Ste24. A, The “ZMP Core” module of Ste24. Functional and structural analyses of homologous, soluble gluzincin orthologs (see Figure 2A) reveal the complete ZMP Core module of Ste24 to be comprised of elements from the CTD (α5‐helix, α7‐helix, and α7‐loop), TM α‐helices (VI and VII), and L5D (β3‐strand and α3‐helix) (see Figure 2B). For clarity, the loop element leading to α7 within CTD is represented by a thin rectangle but does not possess α‐helical secondary structure. Catalytic and substrate specificity regions are indicated by C and S1′, respectively; SmSte24 residues which are absolutely conserved in Ste24 and within these regions are explicitly labeled and bracketed. B, Superposition of the unbound SmSte24 structure with our recently determined structure of HsSte24 complexed with the ZMP inhibitor phosphoramidon (PDB 6BH8).35 For clarity, only the phosphoramidon molecule (shown as green sticks) of the HsSte24:phosphoramidon structure is displayed. The three‐dimensional image of the ZMP Core module is oriented and labeled to match the schematic cartoon presented in panel (a). Residues contributing to the catalytic (C) and specificity (S1′) functions are explicitly labeled, shown as sticks, and their respective surfaces shown. The phosphoramidon inhibitor is bound and coordinated by residues solely within the ZMP Core module of Ste24, in a binding pose analogous to that observed in its complexes with soluble gluzincins.35 C, The Ste24 Tripartite Architecture. The “ZMP Core” module (dark gray ribbon and van der Waals surface) is shown in the three‐dimensional context of the entire Ste24 structure. Individual helix and strand elements constituting the ZMP core are labeled. Analogous to soluble gluzincins, the ZMP Core module is embedded between accessory structural elements (gray ribbon). Such “ZMP Accessory” modules are highly variable in size, scope, and functional role even among highly homologous gluzincins. The ZMP core region is “capped” by a large, membrane‐spanning region, the “α‐barrel” module, whose contribution to Ste24 catalytic activity is currently unknown (light gray ribbon). D, Ste24 topology diagram mapped with the Tripartite Architecture. The color‐coding scheme is maintained from panel C and labeling scheme from Figure 1C