This group of proteins are membrane bound transport proteins essential for ferric ion uptake in bacteria [1]. The Pfam family consists of ExbD, and TolR which are involved in TonB-dependent transport of various receptor bound substrates including colicins [2].

1: Unusual structure of the tonB-exb DNA region of Xanthomonas campestris pv. campestris: tonB, exbB, and exbD1 are essential for ferric iron uptake, but exbD2 is not. Wiggerich HG, Klauke B, Koplin R, Priefer UB, Puhler A; J Bacteriol 1997;179:7103-7110. PMID:9371459

2: Nucleotide sequence of a gene cluster involved in entry of E colicins and single-stranded DNA of infecting filamentous bacteriophages into Escherichia coli. Sun TP, Webster RE; J Bacteriol 1987;169:2667-2674. PMID:3294803