How helminth lipid-binding proteins offload their ligands to membranes: differential mechanisms of fatty acid transfer by the ABA-1 polyprotein allergen and Ov-FAR-1 proteins of nematodes and Sj-FABPc of schistosomes.
Three different classes of small lipid-binding protein (LBP) are found in helminth parasites. Although of similar size, the ABA-1A1 (also designated As-NPA-A1) and Ov-FAR-1 (formerly known as Ov20) proteins of nematodes are mainly alpha-helical and have no known structural counterparts in mammals, whereas Sj-FABPc of schistosomes is predicted to form a beta-barrel structure similar to the mammalian family of intracellular fatty acid binding proteins. The parasites that produce these proteins are unable to synthesize their own complex lipids and, instead, rely entirely upon their hosts for supply. As a first step in elucidating whether these helminth proteins are involved in the acquisition of host lipid, the process by which these LBPs deliver their ligands to acceptor membranes was examined, by comparing the rates and mechanisms of ligand transfer from the proteins to artificial phospholipid vesicles using a fluorescence resonance energy transfer assay. All three proteins bound the fluorescent fatty acid 2-(9-anthroyloxy)palmitic acid (2AP) similarly, but there were clear differences in the rates and mechanisms of fatty acid transfer. Sj-FABPc displayed a collisional mechanism; 2AP transfer rates increased with acceptor membrane concentration, were modulated by acceptor membrane charge, and were not diminished in the presence of increasing salt concentrations. In contrast, transfer of ligand from Ov-FAR-1 and ABA-1A1 involved an aqueous diffusion step; transfer rates from these proteins were not modulated by acceptor membrane concentration or charge but did decrease with the ionic strength of the buffer. Despite these differences, all of the proteins interacted directly with membranes, as determined using a cytochrome c competition assay, although Sj-FABPc interacted to a greater extent than did Ov-FAR-1 or rABA-1A1. Together, these results suggest that Sj-FABPc is most likely to be involved in the intracellular targeted transport and metabolism of fatty acids, whereas Ov-FAR-1 and ABA-1A1 may behave in a manner analogous to that of extracellular LBPs such as serum albumin and plasma retinol binding protein.
McDermott
Lindsay
L
Department of Nutritional Sciences, Rutgers University, New Brunswick, New Jersey 08901, USA. lindsaym@chem.gla.ac.uk
Kennedy
Malcolm W
MW
McManus
Donald P
DP
Bradley
Jan E
JE
Cooper
Alan
A
Storch
Judith
J
D000485:Allergens; D000818:Animals; D000947:Antigens, Helminth / Q000737:chemistry / Q000378:metabolism*; D052179:Antigens, Plant; D030161:Avian Proteins; D001693:Biological Transport, Active; D002352:Carrier Proteins / Q000737:chemistry / Q000145:classification / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids / Q000378:metabolism*; D015801:Helminth Proteins / Q000737:chemistry / Q000378:metabolism*; D006790:Host-Parasite Interactions; D054834:Lipocalins; D009348:Nematoda; D009363:Neoplasm Proteins; D010169:Palmitic Acids / Q000378:metabolism; D011485:Protein Binding; D017433:Protein Structure, Secondary; D011994:Recombinant Proteins / Q000737:chemistry / Q000378:metabolism; D012177:Retinol-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D054839:Retinol-Binding Proteins, Plasma; D012547:Schistosoma
C093689:ABA-1 protein, Ascaris suum; D000485:Allergens; D000947:Antigens, Helminth; D052179:Antigens, Plant; D030161:Avian Proteins; D002352:Carrier Proteins; C066299:Ch21 protein, chicken; D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids; D015801:Helminth Proteins; D054834:Lipocalins; D009363:Neoplasm Proteins; C109308:Ov20 antigen; D010169:Palmitic Acids; D011994:Recombinant Proteins; D012177:Retinol-Binding Proteins; D054839:Retinol-Binding Proteins, Plasma; C098079:2-(9-anthroyloxy)palmitate