The Arabidopsis ALF 4 protein is a regulator of SCF E3 ligases

The cullin‐ RING E3 ligases ( CRL s) regulate diverse cellular processes in all eukaryotes. CRL activity is controlled by several proteins or protein complexes, including NEDD 8, CAND 1, and the CSN . Recently, a mammalian protein called Glomulin ( GLMN ) was shown to inhibit CRL s by binding to the RING BOX ( RBX 1) subunit and preventing binding to the ubiquitin‐conjugating enzyme. Here, we show that Arabidopsis ABERRANT LATERAL ROOT FORMATION 4 ( ALF 4) is an ortholog of GLMN . The alf4 mutant exhibits a phenotype that suggests defects in plant hormone response. We show that ALF 4 binds to RBX 1 and inhibits the activity of SCF TIR 1 , an E3 ligase responsible for degradation of the Aux/ IAA transcriptional repressors. In vivo , the alf4 mutation destabilizes the CUL 1 subunit of the SCF . Reduced CUL 1 levels are associated with increased levels of the Aux/ IAA proteins as well as the DELLA repressors, substrate of SCF SLY 1 . We propose that the alf4 phenotype is partly due to increased levels of the Aux/ IAA and DELLA proteins.