Cleaving in the Sheaves

When Katherine Osteryoung started her postdoctoral fellowship in Elizabeth Vierling's laboratory at the University of Arizona, Tucson, she wanted to work on heat shock proteins, which cells make in response to stress. Chloroplast division was the farthest thing from her mind. But science, which is no stranger to serendipity, had other plans. Plastids, including chloroplasts, are descendants of cyanobacterial endosymbionts. Like their ancestors, they divide by fission, ensuring continuity through succeeding generations and amplification during specialization of tissues such as the photosynthetic cells of leaves. But how plastids split is still a mystery. A number of years ago, two filamentous rings were discovered in the constriction furrow of dividing chloroplasts, one inside the organelle next to the inner of the two bounding membranes and the other directly opposite in the cytoplasm.1 The rings may help power constriction, perhaps by acting as purse strings. The presence of similar filaments in dividing bacteria made the purse string hypothesis even more compelling. The bacterial filaments contain a protein called FtsZ, which is a distant relative of tubulin, the protein building block of cytoskeletal microtubules that guide mitosis and other cell movements.2,3 That's where Osteryoung came in. While working on potential interactions between heat shock proteins and the cytoskeleton, she found a gene for FtsZ in the plant Arabidopsis thaliana.4 With what scientists already knew about chloroplast filaments, bacteria, and FtsZ, the plastid story seemed to come full circle. Osteryoung, now an assistant professor with her own group at the University of Nevada, Reno, has added to the chloroplast division story.5 It turns out that Arabidopsis has a family of AtFtsZ genes, all of which reside in the nucleus (most chloroplast genes moved to the nucleus during evolution). One of the genes encodes the protein AtFtsZ1-1. A short transit peptide attached to one end of the protein brings it into the plastid and then is clipped off, so Osteryoung thinks AtFtsZ1-1 probably comprises the inner filament ring. A second gene doesn't encode a transit peptide, indicating that the protein (AtFtsZ2-1) stays in the cytoplasm. That was a surprise. But when Osteryoung's team synthesized proteins in vitro and incubated them with isolated chloroplasts, they found that only AtFtsZ1-1 got in. Says Oster- young, "It took a long time to be convinced that the second FtsZ is a cytosolic protein." She now thinks it probably builds the outer ring. Both FtsZ proteins are important in division--when either is depleted by antisense technology, plastid division suffers, resulting in many fewer chloroplasts than the normal 100 or so per leaf cell. But fewer also means larger--many of the cells have just one huge chloroplast. The plants have no other obvious abnormalities, according to Osteryoung. When Osteryoung compared AtFtsZ proteins to those of bacteria, they were most similar to cyanobacterial FtsZ, consistent with the endosymbiotic origin of chloroplasts. Plant FtsZ genes have also been found in pea, rice, and a moss.6 Knocking out the moss gene results in giant chloroplasts, as in Arabidopsis. The link between genes, proteins, and filament rings is still circumstantial. Says Osteryoung, we now "have to do localization experiments to see if the two proteins are indeed located on opposite sides of the chloroplast constriction." If they are, the next step is to find out how the two rings are coordinated. To make things even more complicated, Osteryoung's team discovered a third Arabidopsis gene that is very similar to AtFtsZ2-1. And FtsZ proteins aren't the only players in chloroplast division; screens for mutants with altered chloroplast number have turned up other genes.7 Deciphering the interactions should show how one chloroplast becomes two. As for serendipity, Osteryoung still marvels at how it all turned out: "I never would have imagined working on plastid division, but I'm glad. It's a lot of fun." Barry A. Palevitz is a professor of botany at the University of Georgia. He can be reached online at palevitz@dogwood.botany.uga.edu. T. Kuroiwa et al., "The division apparatus of plastids and mitochondria," International Review of Cytology, 181:1-41, 1998. H.P. Erickson, "FtsZ, a tubulin homologue in prokaryotic cell division," Trends in Cell Biology, 7:362-7, 1997. H.P. Erickson, "Atomic structures of tubulin and FtsZ," Trends in Cell Biology, 8:133-7, May 1998. K. W. Osteryoung and E. Vierling, "Conserved cell and organelle division," Nature, 376:473-4, 1995. K.W. Osteryoung et al., "Chloroplast division in higher plants requires members of two functionally divergent gene families with homology to bacterial FtsZ," The Plant Cell, 10:1991-2004, December 1998. R. Strepp et al., "Plant nuclear gene knockout reveals a role in plastid division for the homolog of the bacterial cell division protein FtsZ, an ancestral tubulin," Proceedings of the National Academy of Sciences, 95:4368-73, April 14, 1998. K.W. Osteryoung and K.A. Pyke, "Plastid division: evidence for a prokaryotically derived mechanism," Current Opinion in Plant Biology, 1:475-9, Dec. 1998.

Cleaving in the Sheaves

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