Affinity purification reveals the association of WD40 protein constitutive photomorphogenic 1 with the hetero-oligomeric TCP-1 chaperonin complex in mammalian cells

Constitutive photomorphogenic 1 (COP1), a protein composed of a RING finger, a coiled-coil domain and seven WD40 repeats, functions as an E3 ubiquitin ligase that targets key transcription factors for ubiquitination and degradation in both higher plants and mammalian cells. While COP1 is required for light-mediated development in plants, its mammalian counterpart has been implicated in tumorigenesis. We previously showed that COP1 forms high-molecular-weight complexes in mammalian cells. Here we report our attempts in characterizing the components of the mammalian COP1 complexes by affinity purification combined with mass spectral analysis. We find that both transiently and stably expressed COP1 associates with the hetero-oligomeric TCP-1 chaperonin complex (TRiC), heat shock protein 70 (Hsp70) and BAG-family molecular chaperone regulator-2 (BAG2). In addition, stably expressed COP1 binds to major vault protein (MVP) and translocated promoter region (Tpr). The TRiC/Hsp70 complex is known to interact with and assist in the folding of a number of WD40 proteins in Saccharomyces cerevisiae. The association of WD40 protein COP1 with TRiC/Hsp70 in mammalian cells suggests that facilitating the folding of WD40 proteins may be a conserved function for TRiC/Hsp70 from yeast to mammals.