Preassembly of annulate lamellae in egg extracts inhibits nuclear pore complex formation, but not nuclear membrane assembly.

Annulate lamellae (AL) are cytoplasmic structures containing pore complexes similar in composition and morphology to nuclear envelope pore complexes. We have tested the ability of Xenopus egg extract to support nuclear membrane and pore complex formation when chromatin is added to extract only after annulate lamellae had been allowed to assemble (preincubated extract). We have observed that preassembly of AL does not inhibit the formation of a double membrane surrounding sperm chromatin. However, these double membranes are often distended, do not support accumulation of karyophilic proteins, and do not possess immunologically or morphologically identifiable pore complexes. We have demonstrated that nuclear pore complex assembly and function can be rescued by adding isolated egg vesicles to the preincubated extract. Our data support the conclusion that multiple vesicle populations are utilized in the formation of a nuclear envelope, including: vesicle population(s) which are common to pore formation in nuclear envelopes and annulate lamellae, and vesicle population(s) which seem to be preferentially utilized for nuclear membrane assembly.