Further in vivo studies on the role of the molecular chaperone, Hsp93, in plastid protein import

Summary In Arabidopsis, Hsp93 is encoded by two genes, atHSP93‐V and atHSP93‐III. We identified two T‐DNA mutants for atHSP93‐III: one being a partial ‘knockdown’ (hsp93‐III‐1) and the other a complete ‘knockout’ (hsp93‐III‐2). Homozygotes for both mutants were indistinguishable from wild type. We crossed each mutant to an atHSP93‐V knockout, and identified double mutants with strongly chlorotic phenotypes. This implied redundancy, which was confirmed by the complementation of mildly chlorotic hsp93‐V plants by atHSP93‐III over‐expression. While the hsp93‐V hsp93‐III‐1 mutant was doubly homozygous, the second double mutant was heterozygous for hsp93‐III‐2 (genotype: hsp93‐V/hsp93‐V; +/hsp93‐III‐2). Attempts to identify an hsp93‐V hsp93‐III‐2 double homozygote were unsuccessful, indicating that the Hsp93 pool is essential for viability. Consistently, siliques of the second double mutant contained aborted seeds (because of a block in the zygote–embryo transition) and failed ovules (because of a moderate defect in female gametophytes). Double‐mutant plants were chlorophyll‐deficient, contained under‐developed chloroplasts, and exhibited stunted growth. In import assays using a chimeric pre‐protein (plastocyanin transit peptide fused to dihydrofolate reductase; PC‐DHFR), a clear defect was observed in hsp93‐V hsp93‐III‐1 chloroplasts. Interestingly, while denaturation or stabilization of the DHFR moiety had a strong effect on import efficiency in the wild type, no such effects were observed with double‐mutant (or tic40) chloroplasts. This indicated that pre‐protein unfolding is not rate‐limiting for import into mutant chloroplasts, and suggested that (unlike the situation in mitochondria) the inner membrane import machinery does not contribute to pre‐protein unfolding at the organellar surface.