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The Transmembrane Helix of the Escherichia coli Division Protein FtsI Localizes to the Septal Ring

Mark C. Wissel, Jennifer L. Wendt, Calista J. Mitchell, and David S. Weiss^*

Department of Microbiology, University of Iowa, Iowa City, Iowa

Received 7 August 2004/ Accepted 17 September 2004

FtsI (also called PBP3) of Escherichia coli is a transpeptidase required for synthesis of peptidoglycan in the division septum and is one of about a dozen division proteins that localize to the septal ring. FtsI comprises a short amino-terminal cytoplasmic domain, a single transmembrane helix (TMH), and a large periplasmic domain that encodes the catalytic (transpeptidase) activity. We show here that a 26-amino-acid fragment of FtsI is sufficient to direct green fluorescent protein to the septal ring in cells depleted of wild-type FtsI. This fragment extends from W22 to V47 and corresponds to the TMH. This is a remarkable finding because it is usual for a TMH to target a protein to a site more specific than the membrane. Alanine-scanning mutagenesis of the TMH identified several residues important for septal localization. These residues cluster on one side of an alpha-helix, which we propose interacts directly with another division protein to recruit FtsI to the septal ring.

Present address: Department of Microbiology, Molecular Genetics and Immunology, University of Kansas Medical Center, Kansas City, KS 66160.

Present address: ConjuGon, Inc., Madison, WI 53719.

Present address: 416 Mozart Court, Wheaton, IL 60187.

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