Cell pellets were directly placed into brass planchettes that then were loaded in to the HPM 010 high pressure freezer and fast frozen. Freeze substitution: After freezing, samples (2) and (3) were placed into a Leica EM AFS Freeze substitution (FS) machine (Leica Microsystems, Bannockburn, IL) and incubated at -90 deg C for 24 hours in 0.1 percent tannic acid in acetone. Samples were washed three times with cold acetone (cooled to -90 degrees C) over 5 minutes, and placed in 1 percent OsO4 and 0.1% UA in cold acetone for 72 hours and held at -90 degrees C. After slowly warming to room temperature at 5 degrees C per hour, the specimens were rinsed in pure acetone three times (10 min. at room temperature). Infiltration and embedding in Durcupan resin was subsequently performed at room temperature. Images were acquired using a JEOL4000EX IVEM, magnification: 30000.0; accelerating voltage: 80.0 keV.