1.B.166.  The LbfU Siderophore Porin (LSP) Family 

These proteins are probably all beta-barrel proteins in the outer membranes of Gram-negative bacterial.  They have various names such as DUF3573, LbfU family proteins and FlxA-like family proteins.  Presumably, they allow the entry of certain siderophores into the periplasm of these bacteria. Some members of this family can transport siderophores chelated with non-Fe metals (Al, Cd, Co, Cu, Hg, Mn, Sn, and Zn) (Majewska et al. 2024).

Strains of Francisella tularensis secrete a siderophore in response to iron limitation. Siderophore production is dependent on fslA, the first gene in an operon that appears to encode biosynthetic and export functions for the siderophore. Transcription of the operon is induced under conditions of iron limitation. FslE (544aas; locus FTT0025 in the Schu S4 genome), located downstream of the siderophore biosynthetic genes, is under Fur regulation and is transcribed as part of the fslABCDEF operon. Ramakrishnan et al. (2008) generated an in-frame deletion of FslE and found that the mutant was defective for growth under iron limitation. The mutant was able to secrete a siderophore but was defective in utilization of the siderophore. The fslE gene product has been localized to the outer membrane of F. tularensis strains. FslE may function as a siderophore receptor/porin in F. tularensis. It shows striking sequence similarity to glycine-rich autotransporters and contains a sequence (residue 114-457) that is repeated at least seven times in an autotransporter protein (TC#1.B.12) of 4234aas (CAR57253).

The reaction catalyzed by FslE is:

siderophore (out)siderophore (periplasm)