A TCP1-RELATED MOLECULAR CHAPERONE FROM PLANTS REFOLDS PHYTOCHROME TO ITS PHOTOREVERSIBLE FORM

FOLDING of the major cytoskeletal components in the cytosol of mammalian cells is mediated by interactions with t-complex polypeptide-1 (TCP1) molecular chaperones1-6, a situation analogous to the chaperonin 60-aided folding of polypeptides in bacteria7,8, chloroplasts9,10 and mitochondria11-13. We have purified a TCP1-related molecular chaperone from etiolated oat seedlings that has a unique structure. Although immunologically related to TCP1, and having amino-acid sequence similarity, its quaternary structure is different from animal TCP1 proteins5,6,14. Electron microscopy and image analysis reveals that the chaperone has two stacked rings of six subunits each, and is distinct in size and configuration. The chaperone copurifies with the soluble cytosolic photoreceptor phytochrome15, and can stimulate refolding of denatured phytochrome to a photoactive form in the presence of Mg-ATP. We propose that this protein is the cytosolic chaperone involved in phytochrome biogenesis in plant cells.