Structure of the Z ring associated protein, ZapD, bound to the C-terminal domain of the tubulin-like protein, FtsZ, suggests mechanism of Z ring stabilization through FtsZ crosslinking [Microbiology]

Cell division in most bacteria is mediated by the tubulin-like FtsZ protein, which polymerizes in a GTP-dependent manner to form the cytokinetic Z ring. A diverse repertoire of FtsZ binding proteins affect FtsZ localization and polymerization to ensure correct Z ring formation. Many of these proteins bind the C-terminal domain (CTD) of FtsZ, which serves as a hub for FtsZ regulation. FtsZ ring-associated proteins, ZapA-D (Zaps), are important FtsZ regulatory proteins that stabilize FtsZ assembly and enhance Z ring formation by increasing lateral assembly of FtsZ protofilaments, which then form the Z ring. There are no structures of a Zap protein bound to FtsZ, hence how these proteins affect FtsZ polymerization has been unclear. Recent data showed ZapD binds specifically to the FtsZ CTD. Thus, to obtain insight into the ZapD-CTD interaction and how it may mediate FtsZ protofilament assembly, we determined the Escherichia coli ZapD-FtsZ CTD structure to 2.67 Angstrom resolution. The structure shows that the CTD docks within a hydrophobic cleft in the ZapD helical domain and adopts an unusual structure composed of two turns of helix separated by a proline kink. FtsZ CTD residue Phe377 inserts into the ZapD pocket, anchoring the CTD in place and permitting hydrophobic contacts between FtsZ residues Ile374, Pro375 and Leu378 with ZapD residues Leu74, Trp77, Leu91 and Leu174. The structural findings were supported by mutagenesis coupled with biochemical and in vivo studies. The combined data suggest that ZapD acts as a molecular crosslinking reagent between FtsZ protofilaments to enhance FtsZ assembly.

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