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The bacterial cell division protein FtsZ is a homolog of tubulin, but it has not been determined whether FtsZ polymers are structurally related to the microtubule lattice. In the present study, we have obtained high-resolution electron micrographs of two FtsZ polymers that show remarkable similarity to tubulin polymers. The first is a twodimensional sheet of protofilaments with a lattice very similar to that of the microtubule wall. The second is a miniring, consisting of a single protofilament in a sharply curved, planar conformation. FtsZ minirings are very similar to tubulin rings that are formed upon disassembly of microtubules but are about half the diameter. This suggests that the curved conformation occurs at every FtsZ subunit, but in tubulin rings the conformation occurs at either f3-or a-tubulin subunits but not both. We conclude that the functional polymer of FtsZ in bacterial cell division is a long thin sheet of protofilaments. There is sufficient FtsZ in Escherichia coli to form a protofilament that encircles the cell 20 times. The similarity of polymers formed by FtsZ and tubulin implies that the protofilament sheet is an ancient cytoskeletal system, originally functioning in bacterial cell division and later modified to make microtubules.FtsZ is an abundant bacterial protein essential for cell division in all bacterial species investigated (1). Outside bacteria, FtsZ has recently been discovered in Arabidopsis as a nuclearencoded gene that is transported into chloroplasts (2). A second Arabidopsis FtsZ (partial) sequence is in the dbEST data base (GenBank accession no. Z48464). FtsZ may form the cytoskeletal framework for cell division in all bacteria, as well as chloroplasts and mitochondria.FtsZ has been identified as a homolog of tubulin on the basis of limited but convincing sequence similarity and its binding and hydrolysis of reviewed in ref. 6). Two laboratories have reported assembly of FtsZ into protofilaments and into larger rods or tubular polymers (7,8). The subunit lattice of these polymers was not clearly resolved but appeared different from that of microtubules. Thus, the structural relation of FtsZ polymers to tubulin remained unclear.The most important place to look for structural similarity is at the level of the subunit lattice because it is this lattice that is dictated by the repeating protein-protein contacts. The gross structure of the polymer is less important as an indicator of conserved structure. An open microtubule wall is shown in Fig. IA, and a computer-reconstructed image of the subunit lattice is shown in Fig. 2A. The two-dimensional (2D) lattice consists of subunits, alternating a-and ,3-tubulin, in a parallel array of longitudinal protofilaments. Subunits are 4.0 nm apart along the protofilament, and the protofilaments are 5.1 nm apart laterally (9). If FtsZ polymers are structurally homologous to a microtubule, we should expect to see long, straightThe publication costs of this article were defrayed in part by page charge payment. This article must ther...

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The FtsZ protein is a GTPase that is essential for cell division in Escherichia coli. During cytokinesis, FtsZ loclizes to a ring at the leading edge of septum synthesis. We report the GTP-dependent polymerization of purified FtsZ measured by sedimentation and light scattering. Protein was determined with Bio-Rad protein assay reagent. Actin was cycle purified from rabbit muscle acetone powder (22). RESULTSThe dynamic distribution of FtsZ protein during the cell cycle (18) and the similarities with tubulins in sequence and in GTP binding (19)(20)(21) point to a possible structural role. We tested whether, like tubulin, FtsZ might polymerize in the presence of GTP (23). Indeed, purified FtsZ supplemented with magnesium became gel-like and opalescent within 10 sec of GTP addition, suggesting that polymerization had occurred. Several independent methods were used to confirm this initial observation.GTP-Dependent Sedimentation. Polymerized tubulin and actin sediment rapidly during ultracentrifugation (22,24). In the presence of GTP, almost all of the FtsZ protein was recovered in the pellet (Fig. 1) The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.