Figure 6.

Summary of the architecture of intranuclear membrane structures revealed by FIB/SEM tomography. (A) Model describing infolding formation: The hierarchy of the compartments (1st, 2nd, and 3rd order infolding) could be explained by invagination of membrane into the respective compartment and subsequent membrane fission. Thus, the lumen of 1st and 3rd order infoldings equals the perinuclear space (PNS, blue) and the lumen of 2nd order infoldings the nucleoplasm (white). Non-enveloped capsids were only found in the nucleoplasm and 2nd order infoldings, enveloped capsids only in the lumen of 1st order infoldings; (1a.) It seemed obvious that enveloped capsids result from capsid budding into 1st order infoldings. (1b.) Capsid envelopes might be able to fuse with membranes of 2nd order infoldings, resulting in non-enveloped capsids in 2nd order infoldings; (2.) Alternatively, non-enveloped capsids in 2nd order infoldings could result from capsid trapping during formation of invaginations; (B) We never found tubular segments with capsids. Tubular segments always contained 2nd order infoldings.