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To understand the polymerization dynamics of FtsZ, a bacterial cell division protein similar to tubulin, insight is required into the nature of the nucleotide bound to the polymerized protein. In a previous study, we showed that the FtsZ polymers contain mostly GDP. A recent study challenged this result, suggesting that the polymerized FtsZ is in a GTP-bound state. Here, we show that, when radiolabelled [gamma-32P]-GTP is used to polymerize FtsZ, GTP is hydrolysed instantaneously. The FtsZ polymer contains both GDP and the radiolabelled inorganic phosphate.

Bacteria and archaea usually divide symmetrically by formation of a septum in the middle of the cell. A key event in cell division is the assembly of the FtsZ ring. FtsZ is the prokaryotic homolog of tubulin and forms polymers in the presence of guanine nucleotides. Here, we specifically address the polymerization of FtsZ and the role of nucleotide hydrolysis in polymer formation and stabilization. Recent structural and biochemical results are discussed and a model for FtsZ polymerization, similar to that for tubulin, is presented.

The prokaryotic tubulin homologue FtsZ polymerizes in vitro in a nucleotide dependent fashion. Here we report that replacement of the strictly conserved Asp212 residue of Escherichia coli FtsZ by a Cys or Asn, but not by a Glu residue results in FtsZ that polymerizes with divalent cations in the absence of added GTP. FtsZ D212C and D212N mutants co-purify with GTP as bound nucleotide, providing an explanation for the unusual phenotype. We conclude that D212 plays a critical role in the coordination of a metal ion and the nucleotide at the interface of two FtsZ monomers.

In Escherichia coli, signal recognition particle (SRP)-dependent targeting of inner membrane proteins has been described. In vitro cross-linking studies have demonstrated that short nascent chains exposing a highly hydrophobic targeting signal interact with the SRP. This SRP, assisted by its receptor, FtsY, mediates the transfer to a common translocation site in the inner membrane that contains SecA, SecG, and SecY. Here we describe a further in vitro reconstitution of SRP-mediated membrane insertion in which purified ribosome-nascent chain-SRP complexes are targeted to the purified SecYEG complex contained in proteoliposomes in a process that requires the SRP-receptor FtsY and GTP. We found that in this system SecA and ATP are dispensable for both the transfer of the nascent inner membrane protein FtsQ to SecY and its stable membrane insertion. Release of the SRP from nascent FtsQ also occurred in the absence of SecYEG complex indicating a functional interaction of FtsY with lipids. These data suggest that SRP/FtsY and SecB/SecA constitute distinct targeting routes.