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"Taenia solium" FABPs are hot
Posted by laclette on 01 Nov 2012 at 00:44 GMT
We were gladly surprised when found the report by Kim et al this month in PLOS NTDs, as we just published a similar study  with a difference of only 8 days. We think that the information in both reports complement each other on a neglected topic in "Taenia solium": the fatty acids and lipid metabolism.
We like to comment on a few issues appearing in both reports that could be submitted for a general discussion among PLOS NTD readers:
1. The primary structure of Kim et al’ TsMFABP2 shows several insertions that make it less similar to other reported flatworm’s FABPs, including TsFABP2 and TsFABP1 (reported by us). Most platyhelminths present two FABPs, but we now have three: TsFABP1/TsMFABP1, TsFABP2 and TsMFABP2. This point has to be clarified and perhaps we should look for analogs of this protein in other related parasites.
2. Both reports show comparable functional FA binding assays. We classified TsFABP1 as a member of the iLBP subfamily iv . Results in the report by Kim et al also show low affinities of TsMFABP1 towards unsaturated fatty acids, confirming this proposal. Both FABPs are also responsible for retinol binding in "T. solium". In the light of these findings perhaps TsMFABP2 should be considered as a member of a different subfamily in the iLBP family.
3. Tissue localization studies yielded similar results in both reports: FABPs are localized in the bladder wall, subtegumentary tissue and spiral cannal. This localization in structures that are in close contact with the host, support the notion that FABPs are involved in the uptake and cytoplasmic transport of FAs.
 Illescas O, Carrero JC, Bobes RJ, Flisser A, Rosas G, Laclette JP. Molecular characterization, functional expression, tissue localization and protective potential of a Taenia solium fatty acid-binding protein. Mol Biochem Parasitol. 2012 Oct 17. [Epub ahead of print]
 Hanhoff T, Lücke C, Spener F. Insights into binding of fatty acids by fatty acid binding proteins. Mol Cell Biochem 2002;239(1-2):45-54.
To the Editor
We appreciate the comments and suggestions on our recent publication . We would like to respond on issues given by Dr. Laclette as follows (in bold-face):
1. The primary structure of TsMFABP2 contained several insertions that make it less similar to other reported flatworm’s FABPs. They also raised a question concerning numbers of the platyhelminth FABPs. Most helminths harbored two FABPs while T. solium has three FABPs (TsMFABP1 [HQ259679]/TsFABP1 [JQ929049], TsMFABP2 [HQ259680] and TsFABP2 [JX470484]). This point has to be clarified.
We compared the primary structures of our TsMFABP1 and 2 with TsFABP1 and 2, and recognized that TsFABP1 (JQ929049) was completely identical with TsMFABP1 (HQ259679) [1,2]. Expression of parasitic proteins is temporally regulated in some cases, for example, T. solium hydrophobic ligand binding proteins are stage-specifically activated . Conversely, TsMFABP1 might be constitutively expressed during metacestode and adult stage. TsMFABP2 (45% sequence identity with TsMFABP1) contains two additional loop-like structure compared to other FABPs hitherto identified . It is well known that, despite their varying degrees of sequence identity (approximately 15–70%), FABP family members conserved characteristic β-barrel structure, which consists of 10 anti-parallel β-strands and two α-helices . More importantly, TsMFABP2 tightly conserved several signatures and motifs representative of the FABP such as motifs 1-3, nuclear localization signal and its regulation site, nuclear export signal, hormone-sensitive lipase binding site and phosphylation site for protein kinase C. Together with the isolation of paralogous genes in an individual TsM genome, identification of distinct spot on 2-DE analysis and subsequent biochemical characterization of the recombinant protein clearly indicated that this protein, which is encoded by TsMFABP2, is novel and genuine TsMFABP.
Currently, it is very hard to predict how many genes are responsible for the FABP activity in individual TsM, since its whole genome information is not available. It is apparent that after duplication, each subfamily of the iLBP family has diverged into multiple paralogous genes with different spatial distributions and/or biochemical properties in vertebrates. However, it is unclear whether the ancestral iLBP gene has undergone duplication event and, if any, how many times has occurred in these parasitic cestodes. Whether each of the expanded genes has acquired divergence, which is sufficient to be classified into different subfamilies is presently unclear. Nevertheless, we believe that at least three different genes homologous to FABP are present in parasitic cestodes including T. solium as mentioned above (TsMFABP1/TsFABP1, TsMFABP2 and TsFABP2). Screening of Echinococcus granulosus genome database in the GenBank also retrieved multiple proteins, which could be classified into at least three different FABP groups: EgFABP1 (AAK12096, AAK51437, CAA56765, Q02970 and AAK00579; these proteins shared >98% identity among them), EgFABP2 (AAK12094, AAK12095 and Q9BMK3; these proteins shared 75% sequence identity with EgFABP1 [AAK00579]) and another gene possibly coded for EgFABP (A48578; sequence identity of 93%). Although biochemical propert of the protein (A48578) has not been elucidated, as judged by their homology values, this protein might have FABP activity. We hope future investigations toward isolation and biological characterization of FAPBs, as well as the completion of genome projects, in parasitic organisms would contribute to address the issues regarding the evolutionary history of iLBP family in these invertebrate animals.
2. We classified TsFABP1 as a member of the iLBP subfamily iv due to low affinities towards unsaturated fatty acids. Both FABPs are also responsible for retinol binding in "T. solium". The TsMFABP2 should be considered as a member of a different subfamily in the iLBP family.
Usual cladistic analyses have divided iLBP family into three different subfamilies of FABP such as cellular retinoic acid binding protein (CRABP) and cellular retinol binding protein (CRBP) , although they could also be categorized into four distinct subfamilies based on conformation and biochemical properties . In our paper, we have positioned TsMFABP1 and TsMFABP2 into the FABP subfamily based on their primary structures. At this moment, it is difficult to classify TsMFABP1 and 2 into each of the iLBP subfamilies, since we do not have sufficient information for such categorization. Furthermore, our phylogenetic analyses demonstrated that invertebrate proteins homologous to the iLBP family members form a clade, which is clearly separated from the iLBP family members of vertebrates (Suppl. Fig S1 and Fig 1B of our publication). When we consider the differential ligand affinities and structural variation between TsMFABP1 and TsMFABP2, the gene pool might recently gain structural polymorphism and following functional diversification. More studies, especially in relation to the determination of the binding spectra against diverse hydrophobic ligand would be required for the detailed classification of TsMFABPs and proper evaluation of their biological roles.
3. FABPs are localized in the bladder wall, subtegumentary tissue and spiral cannal. This localization in structures that are in close contact with the host, support the notion that FABPs are involved in the uptake and cytoplasmic transport of FAs.
In our study, TsMFABP1 was localized in the fibrillar stroma of the bladder wall and the spiral canal; TsMFABP2 was more restricted in the canal region surrounding the neck, which correlated well with distribution pattern of retinol. Based on these observations, we proposed that TsMFABP1 might be a counterpart of the TsM 150-kDa HLBP  to convey the trafficking of FAs in the intracellular phase, while TsMFABP2 might have evolved, or is still evolving, to acquire an additional property as a retinol transporter, in certain circumstances, such as high retinol concentrations. During the development of the vertebrates, retinol is profoundly localized in the posterior position. Colocalization of TsMFABP2 and retinol in the canal zone suggests that as-yet undefined neoblasts deposited in the this area might differentiate into diverse cell types during metamorphosis and maturation of the adult worm.
1. Kim SH, Bae YA, Yang HJ, Shin JH, Diaz-Camacho SP, Nawa Y, Kang I, Kong Y (2012) Structural and binding properties of two paralogous fatty acid binding proteins of Taenia solium metacestode. PLoS Negl Trop Dis 6:e1868.
2. Illescas O, Carrero JC, Bobes RJ, Flisser A, Rosas G, Laclette JP (2012) Molecular characterization, functional expression, tissue localization and protective potential of a Taenia solium fatty acid-binding protein. Mol Biochem Parasitol Oct 17. [Epub ahead of print]
3. Rahman M, Lee EG, Kim SH, Bae YA, Wang H, Yang Y, Kong Y (2012) Characterization of hydrophobic-ligand-binding proteins of Taenia solium that are expressed specifically in the adult stage. Parasitology 139:1361-1374.
4. Hertzel AV, Bernlohr DA (2000) The mammalian fatty acid-binding protein multigene family: Molecular and genetic insights into function. Trends Endocrinol Metab 11:175-180.
5. Hanhoff T, Lücke C, Spener F (2002) Insights into binding of fatty acids by fatty acid binding proteins. Mol Cell Biochem 239:45-54.
6. Lee EG, Kim SH, Bae YA, Chung JY, Suh M, et al. (2007) A hydrophobic ligand-binding protein of the Taenia solium metacestode mediates uptake of the host lipid: implication for the maintenance of parasitic cellular homeostasis. Proteomics 7:4016–4030.