Limited proteolysis of chicken gizzard 5'-nucleotidase

Current Approaches to Engineering N-Linked Protein Glycosylation in Bacteria

N-Linked protein glycosylation is a common posttranslational protein modification in eukaryotes involved in many biological processes. As glycosylation is also important for the function and the pharmacokinetic properties of many protein therapeutics, there is an increasing interest in expression systems able to produce glycoproteins of well-defined structure. Bacterial expression hosts generally do not glycosylate proteins at all. However, the discovery of bacterial N-glycosylation systems has opened up a new route for the production of therapeutically interesting glycoproteins in glyco-engineered bacteria. This review offers an introduction to the many efforts taken to engineer bacteria in order to produce N-glycoproteins with defined eukaryotic glycan structures, completely novel protein glycoconjugates as well as to establish screening approaches for improvement and adaptation of the glycosylation machinery to specific applications.

Serine scanning—A tool to prove the consequences of N-glycosylation of proteins

N-Glycosylation of proteins is a common posttranslational modification in eukaryotes. Often this results in enhanced protein stability through protection by the attached sugar moieties. Due to its 13 potential N-glycosylation motifs (N-X-T/S), recombinant hydroxynitrile lyase isoenzyme 5 from almonds (PaHNL5) is secreted by the heterologous host Pichia pastoris in a massively glycosylated form, and it shows extraordinary stability at low pH. The importance of N-glycosylation in general, and individual glycosylation sites in particular for stability at low pH were investigated. To identify especially important glycosylation sites asparagine from all N-X-S/T-motifs was replaced by serine. Thus, critical sites, which contributed to overall enzyme activity and/or stability, were identified individually. One glycosylation site revealed to be essential for stability at low pH. After enzymatic deglycosylation, leaving only one acetylglucosamine attached to asparagines, PaHNL5 retained most of its stability at low pH. Protonation effects in the active site as well as higher-order aggregational events upon incubation in low pH were excluded. This study provides evidence for the interconnection of N-glycosylation and stability at low pH for PaHNL5. Moreover, serine scanning was proven to be applicable for quick identification of critical glycosylation sites.

Characterization of a monoclonal antibody specific for a novel primate cell surface marker with distinct biochemical properties on human erythroleukemia and myeloid cell lines

A hybridoma, CSC-1, which secretes monoclonal antibody (MAb) specific for a cell surface molecule on African green monkey kidney cell line, BGMK, was isolated and characterized. The cell surface molecule recognized by CSC-1 is widely expressed on a variety of human cell lines. Among the hematopoietic cell lines examined, the CSC-1 marker seems to be preferentially expressed by lymphoid cell lines (e.g., Raji, CEM-SS, Jurkat, and MOLT-3). Although CSC-1 also recognizes some uncommitted myeloid and erythroleukemia cell lines (e.g., U-937 and K562), the CSC-1 marker expression on these cell lines is subjected to PMA-induced down-regulation. In addition, the PMA-induced cell surface down-regulation of the CSC-1 marker required more than 24 h. Proteolytic analysis shows that the CSC-1 marker can be categorized into proteinase K-resistant and-sensitive phenotypes. Furthermore, the CSC-1 marker exhibits a slow cell surface turnover rate after proteinase K treatment. Our data suggest that the CSC-1 cell surface molecule might be useful as a megakaryocytic or monocytic differentiation marker.

Asparagine-linked oligosaccharides protect Lamp-1 and Lamp-2 from intracellular proteolysis

Lysosomes contain several integral membrane proteins (termed Lamps and Limps) that are extensively glycosylated with asparagine-linked oligosaccharides. It has been postulated that these glycans protect the underlying polypeptides from the proteolytic environment of the lysosome. Previous attempts to test this hypothesis have been inconclusive because they utilized approaches that prevent initial glycosylation and thereby impair protein folding. We have used endoglycosidase H to remove the Asn-linked glycans from fully folded lysosomal membrane proteins in living cells. Deglycosylation of Lamp-1 and Lamp-2 resulted in their rapid degradation, whereas Limp-2 was relatively stable in the lysosome in the absence of high mannose Asn-linked oligosaccharides. Depletion of Lamp-1 and Lamp-2 had no measurable effect on endosomal/lysosomal pH, osmotic stability, or density, and cell viability was maintained. Transport of endocytosed material to dense lysosomes was delayed in endoglycosidase H treated cells, but the rate of degradation of internalized bovine serum albumin was unchanged. These data provide direct evidence that Asn-linked oligosaccharides protect a subset of lysosomal membrane proteins from proteolytic digestion in intact cells.