Charge Detection Mass Spectrometry Identifies Preferred Non-Icosahedral Polymorphs in the Self-Assembly of Woodchuck Hepatitis Virus Capsids.

Woodchuck hepatitis virus (WHV) is prone to aberrant assembly in vitro and can form a broad distribution of oversized particles. Characterizing aberrant assembly products is challenging because they are both large and heterogeneous. In this work, charge detection mass spectrometry (CDMS) is used to measure the distribution of WHV assembly products. CDMS is a single-particle technique where the masses of individual ions are determined from simultaneous measurement of each ion's charge and m/z (mass-to-charge) ratio. Under relatively aggressive, assembly promoting conditions, roughly half of the WHV assembly products are T=4 capsids composed of exactly 120 dimers while the other half are a broad distribution of larger species that extends to beyond 210 dimers. There are prominent peaks at around 132 dimers and at 150 dimers. In part, the 150 dimer complex can be attributed to elongating a T=4 capsid along its 5-fold axis by adding a ring of hexamers. However, most of the other features cannot be explained by existing models for hexameric defects. Cryo-electron microscopy provides evidence of elongated capsids. However, image analysis reveals that many of them are not closed but have "spiral-like" morphologies. The CDMS data indicate that oversized capsids have a preference for growth by addition of 3 or 4 dimers, probably by completion of hexameric vertices.