Spontaneous Assembly of Pore Complex-Containing Membranes ("Annulate Lamellae") in Xenopus Egg Extract in the Absence of Chromatin

Extract prepared from activated Xenopus eggs is capable of reconstituting nuclei from added DNA or chromatin. We have incubated such extract in the absence of DNA and found that numerous flattened membrane cisternae containing densely spaced pore complexes (annulate lamellae) formed de novo. By electron and immunofluorescence microscopy employing a pore complex-specific antibody we followed their appearance in the extract. Annulate lamellae were first detectable at a 30-min incubation in the form of short cisternae which already contained a high pore density. At 90-120 min they were abundantly present and formed large multilamellar stacks. The kinetics of annulate lamellae assembly were identical to that of nuclear envelope formation after addition of DNA to the extract. However, in the presence of DNA or chromatin, i.e., under conditions promoting the assembly of nuclear envelopes, annulate lamellae formation was considerably reduced and, at sufficiently high chromatin concentrations, completely inhibited. Incubation of the extract with antibodies to lamin LIII did not interfere with annulate lamellae assembly, whereas in the presence of DNA formation of nuclear envelopes around chromatin was inhibited. Our data show that nuclear membrane vesicles are able to fuse spontaneously into membrane cisternae and to assemble pore complexes independently of interactions with chromatin and a lamina. We propose that nuclear envelope precursor material will assemble into a nuclear envelope when chromatin is available for binding the membrane vesicles, and into annulate lamellae when chromatin is absent or its binding sites are saturated.