Lipocalin, bacterial (IPR002446)

Short name: Lipocalin_bac

Overlapping homologous superfamilies

Family relationships


The lipocalins are a diverse, interesting, yet poorly understood family of proteins composed, in the main, of extracellular ligand-binding proteins displaying high specificity for small hydrophobic molecules [PMID: 2580349, PMID: 8761444, PMID: 11058749]. Functions of these proteins include transport of nutrients, control of cell regulation, pheromone transport, cryptic colouration, and the enzymatic synthesis of prostaglandins.

The crystal structures of several lipocalins have been solved and show a novel 8-stranded anti-parallel beta-barrel fold well conserved within the family. Sequence similarity within the family is at a much lower level and would seem to be restricted to conserved disulphides and 3 motifs, which form a juxtaposed cluster that may act as a common cell surface receptor site [PMID: 8761444]. By contrast, at the more variable end of the fold are found an internal ligand binding site and a putative surface for the formation of macromolecular complexes [PMID: 8573354]. The anti-parallel beta-barrel fold is also exploited by the fatty acid-binding proteins (which function similarly by binding small hydrophobic molecules), by avidin and the closely related metalloprotease inhibitors, and by triabin. Similarity at the sequence level, however, is less obvious, being confined to a single short N-terminal motif. The lipocalin family can be subdivided into kernal and outlier sets. The kernal lipocalins form the largest self consistent group. The outlier lipocalins form several smaller distinct subgroups: the OBPs, the von Ebner's gland proteins, alpha-1-acid glycoproteins, tick histamine binding proteins and the nitrophorins.

Relatively recently, bacterial lipocalins have been described for the first time [PMID: 8571450, PMID: 7559452, PMID: 9202455]. These are lipoproteins anchored to the outer membrane of Gram-negative bacteria and some plants. Their promoters are activated at the transition between exponential and stationary growth phases. Bacterial lipocalin sequences are quite closely related to apolipoprotein D and may serve a starvation response function in bacteria. Overexpression, membrane fractionation, and metabolic labelling with tritiated palmitate showed bacterial lipocalins to be globomycin-sensitive outer membrane proteins. The bacterial lipocalins have been found in a small number of species, raising the possibility that they originated by horizontal transfer. Estimates of the G+C content in the first and third codon positions of these genes have been calculated. A biased %G+C in the 1st and 3rd codon positions would suggest horizontal transfer. None of the computed G+C contents of the bacterial lipocalin genes were outside of the expected limits (between the first and third quartiles). These data provide no support for a hypothesis in which bacterial lipocalins were recently acquired through horizontal transfer. Further evidence against horizontal transfer will come from finding more lipocalins in different species, thus making the gene transfer hypothesis more unlikely.

GO terms

Biological Process

No terms assigned in this category.

Molecular Function

GO:0005215 transporter activity

Cellular Component

No terms assigned in this category.

Contributing signatures

Signatures from InterPro member databases are used to construct an entry.