This prokaryotic family of lipoproteins are related to ApbE from Salmonella typhimurium. ApbE is involved in thiamine synthesis [1]. It acts as an FAD:protein FMN-transferase, catalysing the attachment of an FMN residue to a threonine residue of a protein via a phosphoester bond in such bacterial flavoproteins [4].

1: The apbE gene encodes a lipoprotein involved in thiamine synthesis in Salmonella typhimurium. Beck BJ, Downs DM; J Bacteriol 1998;180:885-891. PMID:9473043

2: A periplasmic location is essential for the role of the ApbE lipoprotein in thiamine synthesis in Salmonella typhimurium. Beck BJ, Downs DM; J Bacteriol 1999;181:7285-7290. PMID:10572132

3: Crystal structure of the ApbE protein (TM1553) from Thermotoga maritima at 1.58 A resolution. Han GW, Sri Krishna S, Schwarzenbacher R, McMullan D, Ginalski K, Elsliger MA, Brittain SM, Abdubek P, Agarwalla S, Ambing E, Astakhova T, Axelrod H, Canaves JM, Chiu HJ, DiDonato M, Grzechnik SK, Hale J, Hampton E, Haugen J, Jaroszewski L, Jin KK, Klock Proteins. 2006;64:1083-1090. PMID:16779835

4: Alternative Pyrimidine Biosynthesis Protein ApbE Is a Flavin Transferase Catalyzing Covalent Attachment of FMN to a Threonine Residue in Bacterial Flavoproteins. Bertsova YV, Fadeeva MS, Kostyrko VA, Serebryakova MV, Baykov AA, Bogachev AV; J Biol Chem. 2013;288:14276-14286. PMID:23558683