Structure and ligand binding of As-p18, an extracellular fatty acid binding protein from the eggs of a parasitic nematode.

Marina Ibáñez-Shimabukuro,Malcolm W Kennedy,Betina Córsico,Alan Cooper,Mads Gabrielsen,Brian O Smith,Alan Riboldi-Tunnicliffe,M Florencia Rey-Burusco,Andrew J Roe,Gisela R Franchini,Kate Griffiths

doi:10.1042/bsr20191292

Abstract

Intracellular lipid-binding proteins (iLBPs) of the fatty acid-binding protein (FABP) family of animals transport, mainly fatty acids or retinoids, are confined to the cytosol and have highly similar 3D structures. In contrast, nematodes possess fatty acid-binding proteins (nemFABPs) that are secreted into the perivitelline fluid surrounding their developing embryos. We report structures of As-p18, a nemFABP of the large intestinal roundworm Ascaris suum, with ligand bound, determined using X-ray crystallography and nuclear magnetic resonance spectroscopy. In common with other FABPs, As-p18 comprises a ten β-strand barrel capped by two short α-helices, with the carboxylate head group of oleate tethered in the interior of the protein. However, As-p18 exhibits two distinctive longer loops amongst β-strands not previously seen in a FABP. One of these is adjacent to the presumed ligand entry portal, so it may help to target the protein for efficient loading or unloading of ligand. The second, larger loop is at the opposite end of the molecule and has no equivalent in any iLBP structure yet determined. As-p18 preferentially binds a single 18-carbon fatty acid ligand in its central cavity but in an orientation that differs from iLBPs. The unusual structural features of nemFABPs may relate to resourcing of developing embryos of nematodes.

Highlights

Intracellular lipid binding proteins are a large family of small, ∼15 kDa, cytosolic proteins that include the fatty acid-binding proteins (FABPs) and the retinol- and retinoic acid-binding proteins (CRBPs, cellular retinoic acid-binding protein (CRABP)) [1]
Some FABPs are involved in the activation of nuclear receptors that induce transcription of multiple genes encoding proteins involved in fatty acid and glucose metabolism, as well as cell differentiation [5,6,7], and this appears to be an important function of retinoid-transporting cellular retinol-binding protein (CRBP) and CRABPs [8]. intracellular lipid-binding protein of which FABPs (iLBP) have diverse amino acid sequences, but their 3D structures are highly similar, all comprising a 10-stranded β-barrel with two small α-helices forming a cap over the presumed portal for entry and exit of ligands [1]
The structures were determined by both X-ray crystallography and nuclear magnetic resonance spectroscopy (NMR) revealing the nematode fatty acid-binding protein (nemFABP)’ unique structural features and mode of ligand binding. We find that this protein, and probably all nemFABPs, has a fold similar to that of other iLBPs, but that it possesses two extended loops so far found definitively only in nemFABPs, with the largest of these loops being remote from the presumed portal of entry for ligands and the binding site itself

Summary

Introduction

Intracellular lipid binding proteins (iLBPs) are a large family of small, ∼15 kDa, cytosolic proteins that include the fatty acid-binding proteins (FABPs) and the retinol- and retinoic acid-binding proteins (CRBPs, CRABPs) [1]. Some FABPs are involved in the activation of nuclear receptors that induce transcription of multiple genes encoding proteins involved in fatty acid and glucose metabolism, as well as cell differentiation [5,6,7], and this appears to be an important function of retinoid-transporting CRBPs and CRABPs [8]. Intracellular proteins with almost identical folds to the mammalian iLBPs have been described widely amongst vertebrates and across the metazoan phyla, including insects, crustaceans and arachnids [9,10] Some FABPs are involved in the activation of nuclear receptors that induce transcription of multiple genes encoding proteins involved in fatty acid and glucose metabolism, as well as cell differentiation [5,6,7], and this appears to be an important function of retinoid-transporting CRBPs and CRABPs [8]. iLBPs have diverse amino acid sequences, but their 3D structures are highly similar, all comprising a 10-stranded β-barrel with two small α-helices forming a cap over the presumed portal for entry and exit of ligands [1].

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Results

Conclusion