BACTERIAL-CELL DIVISION PROTEIN FTSZ ASSEMBLES INTO PROTOFILAMENT SHEETS AND MINIRINGS, STRUCTURAL HOMOLOGS OF TUBULIN POLYMERS

The bacterial cell division protein FtsZ is a homolog of tubulin, but it has not been determined whether FtsZ polymers are structurally rela ted to the microtubule lattice. In the present study, we have obtained high-resolution electron micrographs of two FtsZ polymers that show r emarkable similarity to tubulin polymers, The first is a two-dimension al sheet of protofilaments with a lattice very similar to that of the microtubule wall. The second is a miniring, consisting of a single pro tofilament in. a sharply curved, planar conformation, FtsZ minirings a re very similar to tubulin rings that are formed upon disassembly of m icrotubules but are about half the diameter, This suggests that the cu rved conformation occurs at every FtsZ subunit, but in tubulin rings t he conformation occurs at either beta- or alpha-tubulin subunits but n ot both. We conclude that the functional polymer of FtsZ in bacterial cell division is a long thin sheet of protofilaments, There is suffici ent FtsZ in Escherichia coli to form a protofilament that encircles th e cell 20 times. The similarity of polymers formed by FtsZ and tubulin implies that the protofilament sheet is an ancient cytoskeletal syste m, originally functioning in bacterial cell division and later modifie d to make microtubules.