Making Recombinant Glycosylated Proteins (I)

Many mammalian cell membrane-bound and secreted proteins are glycosylated. The degree of their modifications may be dependent upon tissue specificity and cellular states such as normal vases cancerous. The existence of these proteins in bodily fluids, in combination to their relevance to diseases, makes glycosylated proteins good candidates for clinical diagnostics. Understanding the biogenesis, structure, and functions of these proteins will also aid research and prevention of cancers.

Cancer formation is a heterogeneous and complex process, involving many factors and cellular signaling pathways in each type of cancer. There are more than 1,200 potential cancer biomarkers identified in the literature by a 2006 review. We found that ~ 70% of the 1,261 proteins listed in are naturally secreted proteins, and some 40-50% glycosylated. The addition of carbohydrate groups during protein glycosylation to asparagines (N-linked), or threonines or serines (O-linked) residues may result in mono-, disaccharide- or branched oligosaccharide composed of as many as 20 monosaccharide residues. Glycosylation, together with other modifications, often change the apparent molecular mass of a secreted protein to many folds to that predicted by amino acid sequence. Such heavy modifications on the surface of proteins can influence their functions as well as characteristics as antigens or analytes. Studies of glycosylated proteins offer great opportunities for improving cancer diagnostics.

There are increasing demands for these glycosylated human proteins in good quantity, purity and affordability by the scientific community to perform fundamental and clinical studies in relation to cancer. Such proteins cannot be expressed in bacteria or yeast because those cells do not carry out equivalent post-translation modifications (PTM) as in mammalian cells. Although there have been successful attempts to modify yeast cells to produce proteins with certain types of glycans attached, they were designed for expressing a few pharmaceutical proteins and not suitable for expressing a wide variety of cancer markers. Aside from PTM, expressing human proteins in microorganisms may be hindered by their different codon usage preferences and protein folding tendencies.

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Tags: cancer markers, glycosylated proteins, glycosylation, recombinant proteins, serum samples