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

Authors:
Open Access
Publisher website
Google Scholar
Abstract
Summary: 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–β‐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.

This publication has 47 references indexed in Scilit: