Arabidopsis Tubby domain-containing F-box proteins positively regulate immunity by modulating PI4K protein levels

center dot The Tubby domain, named after the TUBBY protein in mice, binds to phosphatidylinositol 4,5-bisphosphate. Arabidopsis has 11 Tubby domain-containing proteins referred to as Tubby-Like Proteins (TLPs). Of the 11 TLPs, 10 possess the N-terminal F-box domain, which can interact with SKP-like proteins and form SKP1-Cullin-F-box E3 ligase complexes. Although mice TUBBY has been extensively studied, plant TLPs' functions are scarcely detailed. center dot In this study, we show that the Arabidopsis Tubby-like protein 6 (TLP6) and its redundant homologs, TLP1, TLP2, TLP5, and TLP10, positively regulate Arabidopsis immune responses. Furthermore, in an immunoprecipitation mass spectrometry analysis to search for ubiquitination substrates of the TLPs, we identified two redundant phosphoinositide biosynthesis enzymes, phosphatidylinositol 4-kinase beta proteins (PI4K beta s), PI4K beta 1 and PI4K beta 2, as TLP interactors. center dot Importantly, TLP6 overexpression lines fully phenocopy the phenotypes of the pi4k beta 1,2 mutant, while TLP6 overexpression also leads to increased PI4K beta 2 ubiquitination and reduction in its protein level in a proteasome-dependent manner. Most significantly, TLP6 overexpression does not further enhance the autoimmunity of the pi4k beta 1,2 double mutant, supporting the hypothesis that TLP6 targets the PI4K beta s for ubiquitination and degradation. center dot Thus, our study reveals a novel mechanism where TLPs promote plant immune responses by modulating the PI4K beta s protein levels.