Recent cases of avian influenza H5N1 and the swine-origin 2009 H1N1 have caused a great concern that a global disaster like the 1918 influenza pandemic may occur again. Viral transmission begins with a critical interaction between hemagglutinin (HA) glycoprotein, which is on the viral coat of influenza, and sialic acid (SA) containing glycans, which are on the host cell surface. To elucidate the role of HA glycosylation in this important interaction, various defined HA glycoforms were prepared, and their binding affinity and specificity were studied by using a synthetic SA microarray. Truncation of the N-glycan structures on HA increased SA binding affinities while decreasing specificity toward disparate SA ligands. The contribution of each monosaccharide and sulfate group within SA ligand structures to HA binding energy was quantitatively dissected. It was found that the sulfate group adds nearly 100-fold (2.04 kcal/mol) in binding energy to fully glycosylated HA, and so does the biantennary glycan to the monoglycosylated HA glycoform. Antibodies raised against HA protein bearing only a single N-linked GlcNAc at each glycosylation site showed better binding affinity and neutralization activity against influenza subtypes than the fully glycosylated HAs elicited. Thus, removal of structurally nonessential glycans on viral surface glycoproteins may be a very effective and general approach for vaccine design against influenza and other human viruses.flu vaccine ͉ glycan binding ͉ glycosylation T he highly pathogenic H5N1 and the 2009 swine-origin influenza A (H1N1) viruses have caused global outbreaks and raised a great concern that further changes in the viruses may occur to bring about a deadly pandemic (1, 2). Important contributions to our understanding of influenza infections have come from the studies on hemagglutinin (HA), a viral coat glycoprotein that binds to specific sialylated glycan receptors in the respiratory tract, allowing the virus to enter the cell (3-6). To cross the species barrier and infect the human population, avian HA must change its receptorbinding preference from a terminally sialylated glycan that contains ␣2,3 (avian)-linked to ␣2,6 (human)-linked sialic acid motifs (7), and this switch could occur through only two mutations, as in the 1918 pandemic (8). Understanding the factors that affect influenza binding to glycan receptors is thus critical for developing methods to control any future crossover influenza strains that have pandemic potential.HA is a homotrimeric transmembrane protein with an ectodomain composed of a globular head and a stem region (3). Both regions carry N-linked oligosaccharides (9), which affect the functional properties of HA (10, 11). Among different subtypes of influenza A viruses, there is extensive variation in the glycosylation sites of the head region, whereas the stem oligosaccharides are more conserved and required for fusion activity (11). Glycans near antigenic peptide epitopes interfere with antibody recognition (12), and glycans near the proteolytic ...

L-Glutamic acid decarboxylase (GAD) exists as both membraneassociated and soluble forms in the mammalian brain. Here, we propose that there is a functional and structural coupling between the synthesis of ␥-aminobutyric acid (GABA) by membraneassociated GAD and its packaging into synaptic vesicles (SVs) by vesicular GABA transporter (VGAT). This notion is supported by the following observations. First, newly synthesized [ 3 H]GABA from [ 3 H]L-glutamate by membrane-associated GAD is taken up preferentially over preexisting GABA by using immunoaffinity-purified GABAergic SVs. Second, the activity of SV-associated GAD and VGAT seems to be coupled because inhibition of GAD also decreases VGAT activity. Third, VGAT and SV-associated Ca 2؉ ͞ calmodulin-dependent kinase II have been found to form a protein complex with GAD. A model is also proposed to link the neuronal stimulation to enhanced synthesis and packaging of GABA into SVs.T he rate-limiting enzyme L-glutamic acid decarboxylase (GAD, EC 4.1.1.15) is involved in the synthesis of ␥-aminobutyric acid (GABA), a major inhibitory neurotransmitter in the mammalian brain. There are two well-characterized GAD isoforms in the human brain, namely GAD 65 and GAD 67 (referring to GAD with a molecular mass of 65 kDa and 67 kDa, respectively) (1). Both GAD 65 and GAD 67 are present as homodimers or heterodimers in soluble GAD (SGAD) and membrane-associated GAD (MGAD) pools (2-4). The ratio of GAD 65 to GAD 67 is higher in synaptic vesicle (SV) fractions than in the cytosol (5). Some studies suggest that GAD 65 binds to the membranes (6, 7) and that GAD 67 subsequently interacts with MGAD 65 (2, 6). However, the nature of anchorage of GAD to membranes and its physiological significance is still not well understood. GAD is not considered to be an integral membrane protein because it lacks a stretch of hydrophobic amino acids long enough to span the membrane. Subpopulations of GAD 65 and GAD 67 remain firmly anchored to membranes despite various ionic extraction methods (2,4,8). The interaction of GAD with membranes was reported to be through ionic (9-11), hydrophobic (12, 13), protein phosphorylation (14), or proteinprotein interaction (15). Previously, we reported that MGAD is activated by phosphorylation that requires an electrochemical gradient across the SV membrane (7). A model for the anchoring mechanism of GAD to SV and its role as a link between GABA synthesis and storage in nerve terminals was also proposed (15). The evidence presented here will demonstrate that GABA synthesized by SV-associated GAD is preferentially transported into the SV by vesicular GABA transporters (VGATs). We have also demonstrated that VGAT, a 10-transmembrane helix protein (16), forms a protein complex with GAD on the SV and could be involved in the anchorage of MGAD to the SV. The formation of this GAD protein complex ensures an efficient coupling between GABA synthesis and packaging into the SV. Materials and MethodsPreparation of SVs. SVs were purified from whole rat brain (Sprague-Dawle...