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* Department of Cell and Molecular Biology, Northwestern University Medical School, Chicago, Illinois 60611; and The nuclear lamina is a fibrous structure that
lies at the interface between the nuclear envelope and the
nucleoplasm. The major proteins comprising the lamina,
the nuclear lamins, are also found in foci in the nucleoplasm, distinct from the peripheral lamina. The nuclear
lamins have been associated with a number of processes in the nucleus, including DNA replication. To
further characterize the specific role of lamins in DNA
replication, we have used a truncated human lamin as a
dominant negative mutant to perturb lamin organization. This protein disrupts the lamin organization of nuclei when microinjected into mammalian cells and also
disrupts the lamin organization of in vitro assembled nuclei when added to Xenopus laevis interphase egg extracts. In both cases, the lamina appears to be completely absent, and instead the endogenous lamins and
the mutant lamin protein are found in nucleoplasmic
aggregates. Coincident with the disruption of lamin organization, there is a dramatic reduction in DNA replication. As a consequence of this disruption, the distributions of PCNA and the large subunit of the RFC
complex, proteins required for the elongation phase of
DNA replication, are altered such that they are found
within the intranucleoplasmic lamin aggregates. In contrast, the distribution of XMCM3, XORC2, and DNA
polymerase
Institut fuer
Biochemie und Molekulare Zellbiologie der Universität Goettingen, D-37073 Goettingen, Germany
, proteins required for the initiation stage
of DNA replication, remains unaltered. The data presented demonstrate that the nuclear lamins may be required for the elongation phase of DNA replication.
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