Biotin Tags as Cryo-EM Tags
Metabolic biotinylation allows one to genetically tag any capsomere of a virus with a small, but detectable electron density that can be observed by imaging. As proof of principle for this approach, the Virology, Vector and Vaccine Engineering Lab tagged the adenovirus protein IX with a 70 amino acid biofilm associated protein (BAP) and performed cryo-electron microscopy (cryo-EM) of this virus.
This work in collaboration with Michael Marsh, M.D., and Wah Chiu, Ph.D., at Baylor College of Medicine led to the reassignment of the location of the IX protein. Rather than being positioned in the 79 angstrom valleys between hexon trimers on the icosahedron, protein IX appears to be located on the surface of the virion between the H2 hexon and the H4 hexon, positioned between adjacent facets, directly above the density previously assigned as protein IIIa.
The original assignment of protein IIIa was based largely on indirect evidence, and the data generated by BAP and other molecular tagging of capsomeres support the reassignment of the protein IIIa density as protein IX. Work is underway to apply this approach for other capsomeres and to apply cryo-EM as an additional method to better understand the biology of native and engineered cell-targeting viruses.