Characterization of a plastid-specific HSP90 homologue: identification of a cDNA sequence, phylogenetic descendence and analysis of its mRNA and protein expression.

The isolation of cDNAs is described which encode the complete sequence of a precursor protein for a HSP90 homologue consisting of an N-terminal transit peptide of 5850 Da and a mature protein (cpHSP82) of 82 260 Da, located in the plastids of rye leaves (Secale cereale). Hybridization analysis indicated the presence of a single gene in the DNA of rye and a transcript size of 2.8 kb. A phylogenetic tree constructed on the basis of sequence comparisons for HSP90 homologues from different species and compartments indicated that the plastidic HSP82 from rye was more closely related to an eubacterial protein than to HSP90 homologues of the cytosol or ER from both plants and animals. The results suggest that during chloroplast evolution the gene for cpHSP82 was transferred to the nucleus from a prokaryotic endosymbiont. Immunoblots with specific antibodies and Percoll gradient-purified organelles confirmed the location of cpHSP82 in chloroplasts or non-green plastids. In green rye leaves cpHSP82 was constitutively expressed and equally distributed among tissues of different age. The expression of cpHSP82 was enhanced within 2 h by exposure to 42 degrees C. The cpHSP82 transcript and protein were much more strongly expressed in non-green tissues, such as etiolated, 70S ribosome-deficient 32 degrees C-grown, or herbicide-bleached, than in normal green leaves. Also chromoplasts from the pericarp of tomato fruits contained high levels of a HSP90 polypeptide while a photosynthetic protein, the large subunit of ribulose-1,5-bisphosphate carboxylase was largely degraded during ripening.