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

FOLDING of the major cytoskeletal components in the cytosol of mammali an cells is mediated by interactions with t-complex polypeptide-1 (TCP 1) molecular chaperones1-6, a situation analogous to the chaperonin 60 -aided folding of polypeptides in bacteria7,8, chloroplasts9,10 and mi tochondria11-13. We have purified a TCP1-related molecular chaperone f rom etiolated oat seedlings that has a unique structure. Although immu nologically related to TCP1, and having amino-acid sequence similarity , its quaternary structure is different from animal TCP1 proteins5,6,1 4. Electron microscopy and image analysis reveals that the chaperone h as two stacked rings of six subunits each, and is distinct in size and configuration. The chaperone copurifies with the soluble cytosolic ph otoreceptor phytochrome15, and can stimulate refolding of denatured ph ytochrome to a photoactive form in the presence of Mg-ATP. We propose that this protein is the cytosolic chaperone involved in phytochrome b iogenesis in plant cells.