Functional characterization of chaperonin containing T-complex polypeptide-1 and its conserved and novel substrates in Arabidopsis

Chaperonin containing T-complex polypeptide-1 (CCT) is an evolutionarily conserved chaperonin multi-subunit complex that mediates protein folding in eukaryotes. It is essential for cell growth and survival in yeast and mammals, with diverse substrate proteins. However, only a few studies on plant CCT have been reported to date, due to the essentiality of CCT subunit genes and the large size of the complex. Here, we have investigated the structure and function of the Arabidopsis CCT complex in detail. The plant CCT consisted of eight subunits that assemble to form a high-molecular-mass protein complex, shown by diverse methods. CCT-deficient cells exhibited depletion of cortical microtubules, accompanied by a reduction in cellular alpha- and beta-tubulin levels due to protein degradation. Cycloheximide-chase assays suggested that CCT is involved in the folding of tubulins in plants. Furthermore, CCT interacted with PPX1, the catalytic subunit of protein phosphatase 4, and may participate in the folding of PPX1 as its substrate. CCT also interacted with Tap46, a regulatory subunit of PP2A family phosphatases, but Tap46 appeared to function in PPX1 stabilization, rather than as a CCT substrate. Collectively, our findings reveal the essential functions of CCT chaperonin in plants and its conserved and novel substrates.