Comparison of the Dynamic Properties of the Bacterial Tubulin Homolog FtsZ, from Bacteria to Chloroplast

Abstract

The tubulin homolog FtsZ is the major cytoskeletal protein in the bacterial cell division machinery, conserved in almost all bacteria, archaea, and chloroplasts. Bacterial FtsZ assembles spontaneously into dynamic protofilaments in vitro and forms a dynamic contractile ring or Z-ring in vivo. FtsZ from different species can have different dynamic properties. Here, we compare the dynamic properties of FtsZ from Escherichia coli (EcFtsZ), from the chloroplast of red alga Galdieria sulphuraria (GsFtsZA and GsFtsZB), and from the cyanobacterium Synechocystis sp. PCC 6803 (SyFtsZ). EcFtsZ assembles in vitro into mostly single protofilaments with an average length of 200 nm, which is highly dynamic, turning over with a half time of 5-8 s both in vitro and in vivo. The mechanism of dynamics is now established to be treadmilling, which involves adding subunits at one end and disassembling at the other. Chloroplasts arose in eukaryotes as endosymbionts of a cyanobacterium. The ancestral ftsZ gene was duplicated early in chloroplast evolution, and most express two FtsZ proteins for its division. Our studies on the GsFtsZA and GsFtsZB showed that both FtsZs assembled into large bundles of protofilaments. Random co-assembly of GsFtsZs enhanced their disassembly kinetics, suggesting GsFtsZB mostly as a regulator. In contrast to the two FtsZs in chloroplasts, there is only one FtsZ in cyanobacteria, similar to other bacteria. Our studies showed SyFtsZ assembled into thick bundles either straight or curved as toroids. This bundling is similar to that of chloroplast FtsZ, consistent with its origin in cyanobacteria. The assembly of SyFtsZ occurred in two stages. The first stage consisted of the assembly of single-stranded protofilaments and opened circles; the second stage formed bundles and toroids. The bundling might benefit the cell division of large cells.