Monoclonal antibodies linked to paramagnetic immunobeads (Dynabeads) have been used to investigate the distribution of lamin B3 in fractions of Xenopus egg extracts. Lamin B3 behaved as if it were completely soluble and did not co-precipitate with membrane fractions. Sperm pronuclei assembled in lamin depleted egg extracts were compared to pronuclei assembled in mock depleted extracts by field emission in-lens electron scanning microscopy (FEISEM). This technique revealed that the surface structures of the nuclear envelopes, including nuclear pores, appeared to be identical, indicating that lamin depletion does not affect nuclear envelope assembly. One-dimensional and two-dimensional gel electrophoresis was used to analyze soluble proteins co-precipitated with lamin B3 on Dynabeads. Our results indicate that two major species (molecular mass: 105 kDa and 57 kDa) specifically co-precipitate with lamin B3 as well as several minor species. At least three proteins which co-precipitate with lamin B3 were identified as nuclear matrix proteins. Lamin B3 was separated from these proteins and re-inoculated into lamin depleted extracts. This resulted in partial rescue of both lamina assembly and DNA replication. These results imply that lamin B3 is directly involved in the assembly of structures required for the initiation of DNA replication.
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