The N-terminal tetra-peptide (IPDE) short extension of the U-box motif in rice SPL11 E3 is essential for the interaction with E2 and ubiquitin-ligase activity.

Rice, a monocot model plant, contains at least 77 U-box E3 ubiquitin (Ub)-ligases and 48 E2 Ub-conjugating enzymes. Here, we investigated the minimal binding domain of rice SPL11 U-box E3 to its E2 partners. Serial deletions and site-directed mutagenesis analyses indicated that, in addition to an intact U-box motif, the N-terminal tetra-peptide (IPDE) short extension of the U-box was essential for the interaction of SPL11 with E2s and Ub-ligase activity. The Ile and Pro residues at the -4 and -3 positions of the U-box, respectively, were crucial for this interaction. These results suggest that the N-terminal tetra-peptide extension of the U-box participates in the specific interaction of SPL11 E3 with E2s in a sequence-specific manner in rice.