Fig. 1.

Crystal structures of SAP05–SPL5^ZnF and SAP05–Rpn10^vWA complexes revealing the bimodular architecture of SAP05. (A) Cartoon model illustrating the fold of the SAP05 effector. α-helices, β-sheets, and loops are indicated in cyan, red, and blue, respectively. (B) Amino acid sequence of SAP05 highlighting the locations of secondary structures (shown in A, color matched) and SPL5^ZnF (purple) and Rpn10^vWA interacting residues (orange) (shown in C–F, color-matched). SPL5^ZnF, zinc-finger domain of SPL5 transcription factors. Rpn10^vWA, von Willebrand factor type A domain of Rpn10 ubiquitin receptor. (C) Crystal structure of the SAP05–SPL5^ZnF complex (PDB 8PFC). The Zn²⁺ ions bound to the two ZnF domains are shown in gray. (D) Crystal structure of the SAP05–Rpn10^vWA complex (PDB 8PFD). In (C) and (D), the dashed lines indicate the interactions between the residues from both components. (E) Interfaces of SAP05 showing the loop surface (purple) and sheet surface (orange) residues that bind to SPL5^ZnF and Rpn10^vWA, respectively. (F) Gel filtration chromatograms of SAP05, SPL5^ZnF, and Rpn10^vWA and binary and ternary complexes of these proteins. The Coomassie-stained protein SDS-PAGE gel shows the presence of the three proteins in the SPL5^ZnF–SAP05–Rpn10^vWA ternary complex—see SI Appendix, Fig. S1, for the other complexes. Elution volumes are indicated at the bottom of each peak with the same colors. (G) Hypothetical ternary structure of SPL5^ZnF–SAP05–Rpn10^vWA obtained by superimposing the crystal structures of SAP05–SPL5^ZnF and SAP05–Rpn10^vWA complexes.