Poster: Photosynthesis (light)

Abs # 216: The Arabidopsis FtsH metalloprotease gene family: interchangeability of subunits in chloroplast oligomeric complexes

Presenter:	Yu, Fei , feiyu@iastate.edu
Authors	Yu, Fei  (A)   Park, Sungsoon  (A)   Rodermel, Steven R (A)
Affiliations:	(A): Department of Genetics, Development and Cell Biology, Iowa State University, Ames, IA 50011

The Arabidopsis FtsH metalloprotease gene family comprises 12 members, including three pairs of closely related genes that are targeted to chloroplasts (AtFtsH2/8; AtFtsH1/5; and AtFtsH7/9). 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 were 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, suggesting interaction between VAR1 and VAR2. Overexpression of AtFtsH8 in var2-4 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. 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 compensating activities in the green sectors. Ongoing experiments will further reveal the relationships between FtsH homologues.