The Arabidopsis COP1/SPA complex acts as a cullin4‐based E3 ubiquitin ligase to suppress photomorphogenesis in darkness. It is a tetrameric complex of two COP1 and two SPA proteins. Both COP1 and SPA are essential for the activity of this complex, and they both contain a C‐terminal WD‐repeat domain responsible for substrate recruitment and binding of DDB1. Here, we used a WD domain swap‐approach to address the cooperativity of COP1 and SPA proteins. We found that expression of a chimeric COP1 carrying the WD‐repeat domain of SPA1 mostly complemented the cop1‐4‐mutant phenotype in darkness, indicating that the WD repeat of SPA1 can replace the WD repeat of COP1. In the light, SPA1‐WD partially substituted for COP1‐WD. In contrast, expression of a chimeric SPA1 protein carrying the WD repeat of COP1 did not rescue the spa‐mutant phenotype. Together, our findings demonstrate that a SPA1‐type WD repeat is essential for COP1/SPA activity, while a COP1‐type WD is in part dispensible. Moreover, a complex with four SPA1‐WDs is more active than a complex with only two SPA1‐WDs. A homology model of SPA1‐WD based on the crystal structure of COP1‐WD uncovered two insertions and several amino acid substitutions at the predicted substrate‐binding pocket of SPA1‐WD.