Mass mapping of a protein complex with the scanning transmission electron microscope

A mass map of the hexagonally packed intermediate layer (HPI-layer), a regular protein monolayer from the cell envelope of Micrococcus radiodurans, has been obtained by scanning transmission electron microscopy. Samples were freeze-dried within the microscope, and low-dose images were recorded in the dark-field mode directly in digital form and processed by correlation averaging. The averaged projection of the unstained structure--i.e., the mass map--thus calculated shows a resolution to 3-nm period and reveals morphological features consistent with those obtained by negative staining. The mass of individual morphological domains was extracted by using variously the mass map itself or an average from a negatively stained HPI layer to define the domain boundaries. Protrusions as small as 1,300 daltons could be measured reproducibly within the unit cell of 655,000 daltons. The method developed opens an avenue to identify molecular species in situ and to correlate topographic information with biochemical data.