The bacterial cell-division protein ZipA and its interaction with an FtsZ fragment revealed by X-ray crystallography

In Escherichia coli, FtsZ, a homologue of eukaryotic tubulins, and ZipA, a membrane-anchored protein that binds to FtsZ, are two essential components of the septal ring structure that mediates cell division. Recent data indic ate that ZipA is involved in the assembly of the ring by linking FtsZ to th e cytoplasmic membrane and that the ZipA-FtsZ interaction is mediated by th eir C-terminal domains. We present the X-ray crystal structures of the C-te rminal FtsZ-binding domain of ZipA and a complex between this domain and a C-terminal fragment of FtsZ, The ZipA domain is a six-stranded beta-sheet p acked against three alpha-helices and contains the split beta-alpha-beta mo tif found in many RNA-binding proteins, The uncovered side of the sheet inc orporates a shallow hydrophobic cavity exposed to solvent. In the complex, the 17-residue FtsZ fragment occupies this entire cavity of ZipA and binds as an extended beta-strand followed by alpha-helix, An alanine-scanning mut agenesis analysis of the FtsZ fragment was also performed, which shows that only a small cluster of the buried FtsZ side chains is critical in binding to ZipA.