21002:

An Arabidopsis variegation locus defines an endosymbiotic FtsH homolog that functions in thylakoid membrane biogenesis.

Authors:	Chen, Meng(A)Choi, YangDo(A)Voytas, Daniel, F. (B)Rodermel, Steve, R. (A)
Affiliations:	(A): Department of Botany, Iowa State University (B): Department of Zoology and Genetics, Iowa State University (C): Interdepartmental Plant Physiology Program, Iowa State University
Presenter:	Rodermel, Steve R. , rodermel@iastate.edu

To gain insight into the factors that regulate chloroplast biogenesis, we have been studying the var2 variegation mutant of Arabidopsis (Martínez-Zapater, 1993, J. Hered. 84: 138-140; Chen et al., 1999, J. Hered. 90: 207-214). Cotyledons in var2 appear normal, but developing true leaves are bright-yellow then turn green-and white sectored. Whereas cells in the green sectors contain morphologically normal chloroplasts, cells in the yellow/white sectors are heteroplastidic and contain plastids with rudimentary lamellar structures, as well as some normal-appearing chloroplasts. This indicates that var2 plastids are affected differently by the nuclear mutation (the mutant is “plastid autonomous”). In contrast to most variegations, defective plastids are not maternally-inherited in var2. At very low light intensities, nearly all-green var2 plants are formed. var2 is not blocked in the steps of carotenoid or chlorophyll biosynthesis. We have recently cloned VAR2 by chromosome walking and have defined the nature of seven var2 alleles. Sequence analyses indicate that VAR2 bears high similarity to the FtsH subfamily of AAA proteins (ATPases associated with diverse cellular activities). VAR2, like other FtsH homologs, has two transmembrane domains at its N-terminus and motifs characteristic of an ATP dependent-zinc protease at its C-terminus. Chloroplast import experiments indicate that VAR2 is localized on the thylakoid membrane with its C-terminus facing the stroma. Northern and Western blots show that VAR2 is expressed only in photosynthetic tissues and that its expression is photoregulated. FtsH mediates a variety of membrane-associated events in prokaryotes and yeast, but its function in multicellular organisms is poorly understood. VAR2 is highly homologous to the red pepper PFTF protein, which promotes vesicle fusion and/or protein translocation events in an in vitro assay using chromoplast vesicles (P. Hugueney et al., 1995, Proc Natl Acad Sci U S A 92: 5630-4). We show that VAR2 functions in thylakoid membrane biogenesis, and that it may be a factor that promotes membrane elaboration from the inner envelope membrane. We postulate that the var2 variegation arises as a consequence of an accumulation of threshold levels of a partially redundant function in some plastids in cells of the developing leaf primordium, and that accumulation of this factor is dependent on cell and plastid division rates.