The Arabidopsis FtsH metalloprotease gene family:: interchangeability of subunits in chloroplast oligomeric complexes

The Arabidopsis At filamentation temperature sensitive (FtsH) metalloprotease gene family comprises 12 members (AtFtsH1-AtFtsH12), including three pairs of closely related genes that are targeted to chloroplasts (AtFtsH2 and AtFtsH8; AtFtsH1 and AtFtsH5; and AtFtsH7 and AtFtsH9). Mutations in AtFtsH5 (var1) and AtFtsH2 (var2) give rise to variegated plants with green- and white-sectored leaves. Cells in the green sectors contain morphologically normal chloroplasts, whereas cells in the white sectors are blocked in chloroplast biogenesis. A major question is how chloroplasts arise in cells that have a mutant genotype. We have found by two-dimensional (2-D) green gel and gel filtration analyses that AtFtsH2/VAR2 forms oligomeric complexes. Two bands in the 2-D green gels that correspond to AtFtsH5/VAR1 + AtFtsH1 and AtFtsH2/VAR2 + AtFtsH8 have been identified, and these bands are coordinately reduced in amount in var1 and var2 thylakoids that lack AtFtsH5/VAR1 and AtFtsH2/VAR2, respectively. These reductions are not because of alterations in transcript abundance. Overexpression of AtFtsH8 in var2-4 (a putative null allele) normalizes the variegation phenotype of the mutant and restores the two bands to their wild-type levels. These results suggest that AtFtsH8 is interchangeable with AtFtsH2/VAR2 in AtFtsH-containing oligomers, and that the two proteins have redundant functions. Consistent with this hypothesis, AtFtsH2 and AtFtsH8 have similar expression patterns, as monitored by promoter-beta-glucuronidase (GUS) fusion and RT-PCR experiments. Based on our findings, we propose that AtFtsH1, AtFtsH2/VAR2, AtFtsH5/VAR1, and AtFtsH8 interact to form oligomeric structures, and that subunit stoichiometry is controlled post-transcriptionally in var1 and var2, perhaps by turnover. A threshold model is presented to explain the pattern of variegation in var2 in which AtFtsH8 provides a compensating activity in the green sectors of the mutant.