Helix packing and subunit conformation in horse spleen apoferritin

An electron density map of horse spleen apoferritin at 0.28-nm (2.8 A) resolution and its preliminary interpretation have been described previously. Rigorous examination of this and newer maps at the same nominal resolution but calculated from more extensive data sets, including model building in a Richards' comparator, now allows us to report on structural features in more detail. We list inter-helical angles within and between neighbouring subunits, and describe a new short region of inter-subunit anti-parallel pleated sheet. A short section of electron density not properly accounted for in the first model has also been found. We also report on two alternative ways of connecting helical regions which account almost equally well for the observed electron density, and we assess these two alternative conformations and compare them with the conformations of other known proteins.