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© The Rockefeller University Press, 0021-9525/1999//1083 $5.00
The Journal of Cell Biology, Volume 144, Number 6, , 1999 1083-1096

Regular Articles

Roles of LAP2 Proteins in Nuclear Assembly and DNA Replication: Truncated LAP2β Proteins Alter Lamina Assembly, Envelope Formation, Nuclear Size, and DNA Replication Efficiency in Xenopus laevis Extracts



Tracey Michele Gant*, Crafford A. Harris{ddagger}, and Katherine L. Wilson*

* Department of Cell Biology and Anatomy, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205; and {ddagger} R.W. Johnson Pharmaceutical Research Institute Drug Discovery, Raritan, New Jersey 08869

Humans express three major splicing isoforms of LAP2, a lamin- and chromatin-binding nuclear protein. LAP2β and {gamma} are integral membrane proteins, whereas {alpha} is intranuclear. When truncated recombinant human LAP2β proteins were added to cell-free Xenopus laevis nuclear assembly reactions at high concentrations, a domain common to all LAP2 isoforms (residues 1–187) inhibited membrane binding to chromatin, whereas the chromatin- and lamin-binding region (residues 1–408) inhibited chromatin expansion. At lower concentrations of the common domain, membranes attached to chromatin with a unique scalloped morphology, but these nuclei neither accumulated lamins nor replicated. At lower concentrations of the chromatin- and lamin-binding region, nuclear envelopes and lamins assembled, but nuclei failed to enlarge and replicated on average 2.5-fold better than controls. This enhancement was not due to rereplication, as shown by density substitution experiments, suggesting the hypothesis that LAP2β is a downstream effector of lamina assembly in promoting replication competence. Overall, our findings suggest that LAP2 proteins mediate membrane–chromatin attachment and lamina assembly, and may promote replication by influencing chromatin structure.

Key Words: nuclear envelope • chromatin structure • prereplication complex • emerin • MAN

Abbreviations used in this paper: BAF, barrier to autointegration factor; BrdU, bromodeoxyuridine; CDK, cyclin-dependent kinase; LAP, lamin-associated polypeptide; LBR, lamin B receptor; MWB, membrane wash buffer; NPC, nuclear pore complex.

T.M. Gant's current address is California Pacific Medical Center, Geraldine Brush Cancer Research Institute, 2330 Clay Street, Stern Building, San Francisco, CA 94115-1932.


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