We are in the process of determining the structure of tubulin using electron crystallography of zinc-induced, crystalline sheets. We have now extended the resolution to 4 Angstrom, and there are many features in the map that appear to show details of the secondary structure. X-ray crystallographers are well aware of the problems of interpreting maps with such limited resolution, and the additional problem of the missing cone of data inherent in electron crystallography may make interpretation even more difficult. To investigate how reliably these maps can be interpreted, we have calculated density maps of a known structure, actin, under conditions similar to those of the tubulin map, Results of these simulations support the limited interpretations we made previously in the 6.5-Angstrom maps and the more extensive interpretations we make here in the 4-Angstrom map, Most of the secondary structure of the tubulin dimer can now be identified. (C) 1997 Academic Press.