Survival of plants in sunlight requires UV-protective responses. UV-B sensing occurs via the UV-B photoreceptor UV RESISTANCE LOCUS 8 (UVR8), which triggers a signaling cascade that leads to plant photomorphogenesis and UV-B acclimation. UVR8 signaling affects plant growth and thus needs to be tightly regulated. In Arabidopsis thaliana, REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1 (RUP1) and RUP2, two highly related WD40-repeat proteins, function as repressors of UVR8 signaling. Despite playing a crucial role in regulating UV-B photomorphogenesis, the molecular mode of action of RUP proteins remains unknown. In this work, new interaction partners for RUP1 and RUP2 were identified and characterized, leading to the discovery of a novel role for RUP proteins in the plant UV-B response.