Restoration of high-level transport activity by human reduced folate carrier/ThTr1 thiamine transporter chimaeras: role of the transmembrane domain 6/7 linker region in reduced folate carrier function

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Abstract
The reduced folate carrier (RFC; SLC19A1) is closely related to the thiamine transporter, SLC19A2 (ThTr1). Hydropathy models for these homologous transporters predict up to 12 transmembrane domains (TMDs), with internally oriented N- and C-termini and a large central loop between TMDs 6 and 7. The homologies are localized mostly in the TMDs. However, there is little similarity in their N- and C-terminal domains and the central peptide linkers connecting putative TMDs 1-6 and TMDs 7-12. To explore the functional role of the 61-amino acid central linker in the human RFC (hRFC), we introduced deletions of 49 and 60 amino acids into this region, differing by the presence of a stretch of 11 highly conserved amino acids between the human and rodent RFCs (positions 204-214). An additional hRFC construct was prepared in which only the 11 conserved amino acids were deleted. The resulting hRFC(D215-R263 Delta), hRFC(K204-R263 Delta) and hRFC(K204-R214 Delta) proteins were transfected into transport-impaired K562 cells. The deletion constructs were all expressed in plasma membranes; however, they were completely inactive for methotrexate and (6 S )5-formyl tetrahydrofolate transport. Insertion of non-homologous 73- and 84-amino acid fragments from the structurally analogous ThTr1 linker region into position 204 of hRFC(K204-R263 Delta) restored low levels of transport (16-21% of the wild type). Insertion of the ThTr1 linkers into hRFC(D215-R263 Delta) at position 215 restored 60-80% of wild-type levels of transport. Collectively, our results suggest that the role of the hRFC linker peptide is to provide the proper spatial orientation between the two halves of the hRFC protein for optimal function, and that this is largely independent of amino acid sequence. Our results also demonstrate a critical transport role for the stretch of 11 conserved amino acids starting at position 204 of hRFC.