Modification of glycoproteins by the attachment of fucose residues is widely distributed in nature. The importance of fucosylation has recently been underlined by identification of the monogenetic inherited human disease "congenital disorder of glycosylation IIc," also termed "leukocyte adhesion deficiency II." Due to defective Golgi GDP-fucose transporter (SLC35C1) activity, patients show a hypofucosylation of glycoproteins and present clinically with mental and growth retardation, persistent leukocytosis, and severe infections. To investigate effects induced by the loss of fucosylated structures in different organs, we generated a mouse model for the disease by inactivating the Golgi GDP-transporter gene (Slc35c1). Lectin binding studies revealed a tremendous reduction of fucosylated glycoconjugates in tissues and isolated cells from Slc35c1 ؊/؊ mice. Fucose treatment of cells from different organs led to partial normalization of the fucosylation state of glycoproteins, thereby indicating an alternative GDP-fucose transport mechanism. Slc35c1-deficient mice presented with severe growth retardation, elevated postnatal mortality rate, dilatation of lung alveoles, and hypocellular lymph nodes. In vitro and in vivo leukocyte adhesion and rolling assays revealed a severe impairment of P-, E-, and L-selectin ligand function. The diversity of these phenotypic aspects demonstrates the broad general impact of fucosylation in the mammalian organism.The covalent attachment of oligosaccharide moieties to newly synthesized proteins comprises one of the most frequent but also complex forms of co-and posttranslational protein modifications, which has been found in nearly all living organisms (1). Glycan structures affect the physicochemical properties and the function of proteins in a variety of biological processes, including folding, solubility, sorting, proteolytic stability, and receptor-ligand interactions. In mammalian organisms, the biosynthesis of the oligosaccharide chains requires a broad spectrum of glycosyltransferases, glycosidases, transport proteins, and 13 different monosaccharides (2-4).Due to its variability of binding types (␣-1,2-, ␣-1,3-, ␣-1,4-, and ␣-1,6-fucosylation have been described), the monosaccharide fucose plays an important role in the microheterogeneity of oligosaccharide structures (5). Fucose residues are predominantly linked to peripheral parts of N-, O-, and lipid-linked oligosaccharides, thereby building cap structures, which have been observed in many surface-localized and secreted proteins,

Leukocyte adhesion deficiency II (LAD II)is a rare congenital disease caused by defective fucosylation leading to immunodeficiency and psychomotor retardation. We have previously identified the genetic defect of LAD II in a patient whose Golgi GDP-fucose transporter (GFTP) bears a single amino acid exchange that renders this protein nonfunctional but correctly localized to the Golgi. We now report a novel dual defect by which a truncated GFTP causes the disease in a new LAD II patient. We show that the truncation renders this GFTP unable to localize to the Golgi, the compartment where it is required. Furthermore, the missing part of the GFTP can be dissected into 2 regions, one that is needed for Golgi localization and one that is additionally required for the function of the GFTP. We investigated the subcellular localization of all known defective GFTPs allowing us to divide all genetically analyzed LAD II patients into 2 groups, one in which single amino acid exchanges in the GFTP impair its function but not its subcellular localization, and another group with a dual defect in function and Golgi expression of the GFTP due to the absence of 2 important molecular regions. IntroductionLeukocyte adhesion deficiency II (LAD II) is a congenital disorder of glycosylation (CDG-IIc) that affects leukocyte interactions with blood-vessel endothelium. These interactions are required for the extravasation of leukocytes to secondary lymphoid organs and sites of infection and involve a cascade of binding events that are initiated by selectin-dependent leukocyte tethering and rolling. 1,2 Selectins are a group of C-type lectin adhesion molecules that bind to ligands that are decorated with fucosylated glycostructures similar or identical to sialyl Lewis X. 3,4 Patients with LAD II display a generalized defect in fucosylation that affects the selectin ligands, strongly impairing leukocyte-endothelial-cell interactions. This causes reduced extravasation of neutrophils and immunodeficiency. The fucosylation defect also leads to prominent neutrophilia as well as mental and growth retardation. [5][6][7][8] We and others have identified mutations in the gene that codes for the GDP-fucose transporter (GFTP) as the genetic defect causing the disease. 9,10 The GFTP resides in the Golgi membrane, where it serves to transport the nucleotide sugar GDP-fucose into the Golgi lumen. 9,11 GDP-fucose is synthesized in the cytosol mainly from mannose and glucose via a de novo pathway and, to a minor extent, via a salvage pathway that uses either exogenous L-fucose or fucose derived from degraded glycoconjugates. 12,13 Following transport into the Golgi by the GFTP, the nucleotide sugar serves as a substrate for fucosylation reactions carried out by several fucosyltransferases. The human GFTP is a protein of 364 amino acids with 10 transmembrane (TM) domains and the amino and carboxy termini exposed to the cytosol 9,10 ( Figure 2). The protein is expressed as a homodimer and translocates GDP-fucose into the Golgi lumen in exchange for GMP.So f...