Microtubule-dependent assembly of the nuclear envelope in Xenopus laevis egg extract.

Microtubules take part in several mechanisms of intracellular motility, including organelle transport and mitosis. We have studied the ability of Xenopus egg extract to support nuclear membrane and pore complex formation when microtubule dynamics are manipulated. In this report we show that the formation of a nuclear envelope surrounding sperm chromatin requires polymerized microtubules. We have observed that microtubule-depolymerizing reagents, and AS-2, a known inhibitor of the microtubule motor protein kinesin, do not inhibit the formation of a double nuclear membrane. However these double membranes contain no morphologically identifiable nuclear pore complexes and do not support the accumulation of karyophilic proteins. In contrast, the assembly of annulate lamellae, cytoplasmic structures containing a subset of pore complex proteins, was not affected. Our data show that not only polymerized microtubules, but also the microtubule motor protein kinesin, are involved in the formation of the nuclear envelope. These results support the conclusion that multiple nuclear envelope-forming mitotic vesicle populations exist, that microtubules play an essential and selective role in the transport of nuclear envelope-forming vesicle population(s), and that separate mechanisms are involved in nuclear envelope and annulate lamellae formation.