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Elsevier, Biochimica et Biophysica Acta - Molecular and Cell Biology of Lipids, 2(1831), p. 417-427, 2013

DOI: 10.1016/j.bbalip.2012.10.010

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The glycolipid transfer protein (GLTP) domain of phosphoinositol 4-phosphate adaptor protein-2 (FAPP2): Structure drives preference for simple neutral glycosphingolipids

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This paper is available in a repository.

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Abstract

Phosphoinositol 4-phosphate adaptor protein-2 (FAPP2) plays a key role in glycosphingolipid (GSL) production using its C-terminal domain to transport newly synthesized glucosylceramide away from the cytosol-facing glucosylceramide synthase in the cis-Golgi for further anabolic processing. Structural homology modeling against human glycolipid transfer protein (GLTP) predicts a GLTP-fold for FAPP2 C-terminal domain, but no experimental support exists to warrant inclusion in the GLTP superfamily. Here, the biophysical properties and glycolipid transfer specificity of FAPP2-C-terminal domain have been characterized and compared with other established GLTP-folds. Experimental evidence for a GLTP-fold includes: i) Far-UV circular dichroism (CD) showing secondary structure with high alpha-helix content and a low thermally-induced unfolding transition (~41°C); ii) Near-UV-CD indicating only subtle tertiary conformational change before/after interaction with membranes containing/lacking glycolipid; iii) Red-shifted tryptophan (Trp) emission wavelength maximum (λ(max) ~352nm) for apo-FAPP2-C-terminal domain consistent with surface exposed intrinsic Trp residues; iv) 'Signature' GLTP-fold Trp fluorescence response, i.e., intensity decrease (~30%) accompanied by strongly blue-shifted λ(max) (~14nm) upon interaction with membranes containing glycolipid, supporting direct involvement of Trp in glycolipid binding and enabling estimation of partitioning affinities. A structurally-based preference for other simple uncharged GSLs, in addition to glucosylceramide, makes human FAPP2-GLTP more similar to fungal HET-C2 than to plant AtGLTP1 (glucosylceramide-specific) or to broadly GSL-selective human GLTP. These findings along with the distinct mRNA exon/intron organizations originating from single-copy genes on separate human chromosomes suggest adaptive evolutionary divergence by these two GLTP-folds.