Question

Cells have oligosaccharides displayed on their cell surface that are important for cell-cell recognition. Your friend discovered a transmembrane glycoprotein, GP1, on a pathogenic yeast cell that is recognized by human immune cells. He decides to purify large amounts of GP1 by expressing it in bacteria. To his purified protein he then adds a branched 14-sugar oligosaccharide to the asparagine of the only Asn-X-Ser sequence found on GP1 (Figure Q15-48). Unfortunately, immune cells do not seem to recognize this synthesized glycoprotein. Which of the following statements is a likely explanation for this problem?

(a) The oligosaccharide should have been added to the serine instead of the asparagine.
(b) The oligosaccharide should have been added one sugar at a time.
(c) The oligosaccharide needs to be further modified before it is mature.
(d) The oligosaccharide needs a disulfide bond.

Answer 1

The synthesized glycoprotein GP1 not being recognized by human immune cells is likely due to a lack of necessary post-translational modification, such as those that occur in the Golgi apparatus of eukaryotic cells, which are not present in bacteria.

Step-by-step explanation:

The most likely explanation for the synthesized glycoprotein GP1 not being recognized by human immune cells is that the oligosaccharide needs to be further modified before it is mature (option c). In eukaryotic systems, glycoprotein synthesis involves an intricate process starting in the rough endoplasmic reticulum (RER), where a core glycoside is attached to compatible amino acids on a membrane protein.

These partially glycosylated proteins travel through the Golgi apparatus, where additional sugars are attached during terminal glycosylation, rendering the glycoprotein fully mature. In this student's case, bacterial expression system was used for protein synthesis, which lacks the complex post-translational modification system of eukaryotic cells. Consequently, the glycoprotein GP1 may not undergo the necessary modifications such as trimming and addition of complex sugar moieties, which are critical for proper recognition by human immune cells.