Prokaryotic Cytoskeletons

Abstract

FtsZ assembles in vitro into protofilaments (pfs) that are one subunit thick and ~50 subunits long. In vivo these pfs assemble further into the Z ring, which, along with accessory division proteins, constricts to divide the cell. We have reconstituted Z rings in liposomes in vitro, using pure FtsZ that was modified with a membrane targeting sequence to directly bind the membrane. This FtsZ-mts assembled Z rings and constricted the liposomes without any accessory proteins. We proposed that the force for constriction was generated by a conformational change from straight to curved pfs. Evidence supporting this mechanism came from switching the membrane tether to the opposite side of the pf. These switched-tether pfs assembled " inside-out " Z rings, and squeezed the liposomes from the outside, as expected for the bending model. We propose three steps for the full process of cyto-kinesis: (a) pf bending generates a constriction force on the inner membrane, but the rigid peptidoglycan wall initially prevents any invagination; (b) downstream pro-teins associate to the Z ring and remodel the peptidoglycan, permitting it to follow the constricting FtsZ to a diameter of ~250 nm; the final steps of closure of the septum and membrane fusion are achieved by excess membrane synthesis and membrane fluctuations. Keywords E. coli • Z-ring constriction • FtsZ • Tubulin • Curved protofilaments • Intermediate curved pfs • Bacterial cell division • Constriction force • Liposomes • FtsZ-MTS • Reconstituted systems • FtsA • Substructure of Z ring • Final step of septum closure • Copy number of divisome proteins