Charge detection mass spectrometry of bacteriophage P22 procapsid distributions above 20 MDa.

RATIONALE Charge state resolution is required to determine the masses of ions in electrospray mass spectrometry, a feat which becomes increasingly difficult as the mass increases. Charge detection mass spectrometry (CDMS) circumvents this limitation by simultaneously measuring the charge and the m/z of individual ions. In this work, we have used electrospray CDMS to determine the number of scaffolding proteins associated with bacteriophage P22 procapsids. METHODS P22 procapsids containing a native cargo of scaffolding protein were assembled in E. coli and purified via differential centrifugation. Electrospray CDMS was used to measure their mass distribution. RESULTS The procapsid peak was centered at 23.60 MDa, which indicates that they contain an average of ~112 scaffolding proteins. The distribution is relatively narrow, less than 31 scaffolding proteins wide. In addition, a peak at 19.84 MDa with a relative abundance of ~15% is attributed to empty capsids. Despite having the same sizes in solution, the empty capsid and the procapsid have significantly different average charges. CONCLUSIONS The detection of empty capsids is unexpected and the process that leads to them is unknown. The average charge on the empty capsids is significantly lower than expected from the charge residue model, which probably indicates that the empty capsids have contracted in the gas phase. The scaffolding protein presumably limits the contraction of the procapsids. This work shows that electrospray CDMS can provide valuable information for masses greater than 20 MDa.