Hi Mike, An icosahedral capsid has twenty 3-fold symmetry axes. Are the 3-fold symmetric spikes aligned with the 3-fold symmetry axes of the icosahedron? If they are then imposing the icosahedral symmetry will make the antibody binding to each spike 3-fold symmetric. If the spike 3-fold symmetry axes are not aligned with the icosahedron 3-fold symmetry axes (seems unlikely) then you would need to know where the spike 3-fold symmetry axes are and in that case I’d suggest fitting an antibody to a single spike using C3 symmetry with an explicitly specified axis vector. Tom

On Dec 26, 2018, at 4:43 PM, Kent, Michael S wrote:

I have an EM map for a virus that includes an antibody bound to the envelope proteins. The envelope proteins are trimers of dimers with C3 symmetry that form spikes on the surface of the virus. I fit an atomic model for the spikes to the map using icosahedral symmetry, following the very helpful symmetric fitting video tutorial <http://www.cgl.ucsf.edu/chimera/videodoc/SymFitDNAb/>. Now I am trying to fit a model for the antibody into the spike regions of the map. I need to impose C3 symmetry for the antibody for the fit into each spike, in addition to imposing icosahedral symmetry to include all the spikes. How can I impose both of these in the fit?

Thanks,

​Mike Kent

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