Cryo electron microscopy structures of Hsp100 proteins: crowbars in or out?

Independent cryo electron microscopy (cryo-EM) studies of the closely related protein disaggregases ClpB and Hsp104 have resulted in two different models of subunit arrangement in the active hexamer. We compare the EM maps and resulting atomic structure fits, discuss their differences, and relate them to published experimental information in an attempt to discriminate between models. In addition, we present some general assessment criteria for low-resolution cryo-EM maps to offer non-structural biologists tools to evaluate these structures.