Which expression system provides authentic post-translational modifications?

• A. Bacterial (E. coli)
• B. Yeast (Saccharomyces)
• C. Plant cells
• D. Mammalian cells

Answer: (D)

Authentic post-translational modifications come from using a system that has the full human-like secretory and processing machinery. Mammalian cells possess the endoplasmic reticulum and Golgi apparatus with the complete set of enzymes needed to add, trim, and produce human-like glycans, form disulfide bonds correctly, and perform other refinements such as precise proteolytic processing, sialylation, and relevant phosphorylation events. This native-like processing is crucial for the structure, stability, activity, and safety of many therapeutic proteins.

Bacteria lack the secretory pathway needed for complex glycosylation and disulfide-bond formation patterns seen in mammalian proteins, so their PTMs are limited and frequently non-human. Yeast can perform glycosylation and disulfide formation but tends to add high-mannose glycans and can produce glycan structures different from human ones. Plant cells also modify proteins, but their glycoforms include plant-specific patterns that can be immunogenic in humans. Therefore, mammalian cells best provide authentic, human-like post-translational modifications.