RESUMO
Many secreted or cell surface proteins are post-translationally modified by carbohydrate chains which are a primary source of heterogeneity. The Lec1 mutant, which is defective in Golgi N-acetylglucosaminyltransferase I (GnTI) activity, produces relatively homogeneous Man(5) GlcNAc(2) glycan modifications, and is widely used for various applications. To facilitate the investigation of GnTI, its Man5 glycan endproduct, and the impact of Man5 on effector function, the present study has established several novel Lec1 mutants in dhfr(-) CHO-DUKX cells through chemical mutagenesis and lectin selection. A total of nine clonal lines exhibiting the Lec1-phenotype are characterized, six of which harbor non-sense mutations leading to a truncated GnTI, and three (R415K, D291N, and P138L) of which are novel loss-of-function sense mutations. Analysis of the rabbit GnTI structure (Unligil et al., 2000) indicates that D291 is the proposed catalytic base and R415 is a crucial residue in forming the substrate binding pocket, whereas P138 is key to maintaining two ß strands in proximity to the substrate binding pocket. Computational modeling reveals that the oligomannose glycan backbone of a glycoprotein (the acceptor substrate) fits nicely into the unoccupied channel of the substrate binding pocket partly through hydrogen bonding with R415 and D291. This finding is consistent with the ordered sequential Bi Bi kinetic mechanism suggested for GnTI, in which binding of UDP-GlcNAc (the donor substrate)/Mn(2+) induces conformational changes that promote acceptor binding. When an anti-human CD20 antibody protein is stably expressed in one CHO-DUKX-Lec1 line, it is confirmed that N-glycans are predominantly Man(5) GlcNAc(2) and they do not contain an α1,6-fucose linked to the innermost GlcNAc. Furthermore, this Man(5) GlcNAc(2) modified antibody exhibits a significantly increased ADCC activity than the wild-type protein, while displaying a lower CDC activity. The data support the hypothesis that modulating GnTI activity can influence antibody effector functions for proteins with an IgG1 immunoglobulin Fc domain.
Assuntos
Mutação , N-Acetilglucosaminiltransferases/genética , Oligossacarídeos/metabolismo , Engenharia de Proteínas/métodos , Sequência de Aminoácidos , Animais , Células CHO , Linhagem Celular , Células Clonais , Cricetinae , Glicosilação , Humanos , Imunoglobulina G/genética , Imunoglobulina G/metabolismo , Modelos Moleculares , Dados de Sequência Molecular , N-Acetilglucosaminiltransferases/química , N-Acetilglucosaminiltransferases/metabolismo , Oligossacarídeos/genética , Coelhos , Proteínas Recombinantes/genética , Proteínas Recombinantes/metabolismo , Alinhamento de SequênciaRESUMO
Acidic mammalian chitinase (AMCase) is a member of the glycosyl hydrolase 18 family (EC 3.2.1.14) that has been implicated in the pathophysiology of allergic airway disease such as asthma. Small molecule inhibitors of AMCase were identified using a combination of high-throughput screening, fragment screening, and virtual screening techniques and characterized by enzyme inhibition and NMR and Biacore binding experiments. X-ray structures of the inhibitors in complex with AMCase revealed that the larger more potent HTS hits, e.g. 5-(4-(2-(4-bromophenoxy)ethyl)piperazine-1-yl)-1H-1,2,4-triazol-3-amine 1, spanned from the active site pocket to a hydrophobic pocket. Smaller fragments identified by FBS occupy both these pockets independently and suggest potential strategies for linking fragments. Compound 1 is a 200 nM AMCase inhibitor which reduced AMCase enzymatic activity in the bronchoalveolar lavage fluid in allergen-challenged mice after oral dosing.