Rare lipoprotein A (RlpA) contains a conserved region that has the double-psi beta-barrel (DPBB) fold [3,4]. The function of RlpA is not well understood, but it has been shown to act as a prc mutant suppressor in Escherichia coli [1]. The DPBB fold is often an enzymatic domain. The members of this family are quite diverse, and if catalytic this family may contain several different functions. Another example of this domain is found in the N terminus of pollen allergen. Recent studies show that the full-length RlpA protein from Pseudomonas Aeruginosa is an outer membrane protein that is a lytic transglycolase with specificity for peptidoglycan lacking stem peptides. Residue D157 in UniProtKB:Q9X6V6 is critical for lytic activity [5].

1: Multicopy suppressors of prc mutant Escherichia coli include two HtrA (DegP) protease homologs (HhoAB), DksA, and a truncated R1pA. Bass S, Gu Q, Christen A; J Bacteriol 1996;178:1154-1161. PMID:8576052

2: Genes encoding two lipoproteins in the leuS-dacA region of the Escherichia coli chromosome. Takase I, Ishino F, Wachi M, Kamata H, Doi M, Asoh S, Matsuzawa H, Ohta T, Matsuhashi M; J Bacteriol 1987;169:5692-5699. PMID:3316191

3: N-ethylmaleimide-sensitive fusion protein (NSF) and CDC48 confirmed as members of the double-psi beta-barrel aspartate decarboxylase/formate dehydrogenase family. Mizuguchi K, Dhanaraj V, Blundell TL, Murzin AG; Structure Fold Des. 1999;7:215-216. PMID:10610264

4: A six-stranded double-psi beta barrel is shared by several protein superfamilies. Castillo RM, Mizuguchi K, Dhanaraj V, Albert A, Blundell TL, Murzin AG; Structure Fold Des 1999;7:227-236. PMID:10368289

5: The bacterial septal ring protein RlpA is a lytic transglycosylase that contributes to rod shape and daughter cell separation in Pseudomonas aeruginosa. Jorgenson MA, Chen Y, Yahashiri A, Popham DL, Weiss DS; Mol Microbiol. 2014;93:113-128. PMID:24806796