Genetic analysis of CAND1–CUL1 interactions in Arabidopsis supports a role for CAND1-mediated cycling of the SCF TIR1 complex

SKP1-Cullin1-F-box protein (SCF) ubiquitin-ligases regulate numerous aspects of eukaryotic growth and development. Cullin-Associated and Neddylation-Dissociated (CAND1) modulates SCF function through its interactions with the CUL1 subunit. Although biochemical studies with human CAND1 suggested that CAND1 plays a negative regulatory role by sequestering CUL1 and preventing SCF complex assembly, genetic studies inArabidopsis have shown thatcand1 mutants exhibit reduced SCF activity, demonstrating thatCAND1 is required for optimal SCF functionin vivo . Together, these genetic and biochemical studies have suggested a model of CAND1-mediated cycles of SCF complex assembly and disassembly. Here, using the SCFTIR1 complex of theArabidopsis auxin response pathway, we test the SCF cycling model withArabidopsis mutant derivatives of CAND1 and CUL1 that have opposing effects on the CAND1–CUL1 interaction. We find that the disruption of the CAND1–CUL1 interaction results in an increased abundance of assembled SCFTIR1 complex. In contrast, stabilization of the CAND1–CUL1 interaction diminishes SCFTIR1 complex abundance. The fact that both decreased and increased CAND1–CUL1 interactions result in reduced SCFTIR1 activityin vivo strongly supports the hypothesis that CAND1-mediated cycling is required for optimal SCF function.