Glycosylation of the capsid proteins of cowpea mosaic virus: a reinvestigation shows the absence of sugar residues

This study reinvestigates claims that cowpea mosaic virus (CPMV) capsid proteins contain sugar residues. Using modern analytical techniques, researchers analyzed purified CPMV particles for monosaccharides. Two independent methods—OPA derivatization detecting amino sugars and PMP derivatization detecting all sugars—revealed no significant sugar content in the viral capsids. The levels of glucosamine and galactosamine, previously reported at 0.70 and 0.36 g/100g protein respectively, were undetectable or at trace levels below 0.05 g/100g protein. Minor traces of other sugars detected were likely contamination from plant material. These findings contradict a 1974 report and suggest the earlier high sugar readings may have resulted from contamination during purification using sucrose gradients. The authors conclude CPMV coat proteins are not glycosylated, which has implications for using CPMV-based particles as vaccine platforms and contradicts previous hypotheses linking glycosylation to seed transmissibility.

Key findings

• Modern analytical methods (OPA and PMP derivatization with RP-HPLC) detected no significant sugar residues in CPMV capsid proteins, contradicting a 1974 report of substantial glycosylation
• Glucosamine and galactosamine, previously reported as major components (0.70 and 0.36 g/100g protein), were undetectable or found only in trace amounts (<0.05 g/100g protein)
• Crystallographic and mass spectrometry data from previous studies support the absence of glycosylation on CPMV particles
• The earlier high sugar measurements were likely due to contamination during virus purification, possibly from sucrose used in gradient electrophoresis
• CPMV-based vaccine candidates should not trigger unwanted immune reactions from plant-specific glycans, and glycosylation does not determine seed transmissibility as previously hypothesized