Organization and modulation of nuclear lamina structure

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The nuclear lamina is a protein meshwork supporting the nuclear envelope and organizing chromosomes.
• Its precise structural organization and dynamic behavior during the cell cycle are not fully understood.

Purpose of the Study:

• To investigate the structural organization of the nuclear lamina.
• To understand the synthesis, assembly, and regulation of lamins during the cell cycle, particularly during cell division.

Main Methods:

• Chemical extraction procedures on rat liver nuclear envelopes to isolate lamina-enriched fractions.
• Analysis of lamin synthesis and assembly in synchronized Chinese hamster ovary cells.
• Two-dimensional gel electrophoresis to analyze charge-altering modifications of lamins during mitosis.

Main Results:

• A nuclear envelope subfraction highly enriched in lamins A, B, and C, devoid of pore complexes, was defined.
• Lamin B shows stronger interaction with nuclear membranes, suggesting a role in lamina attachment.
• Lamins are synthesized at similar rates throughout the cell cycle and rapidly assemble into the insoluble lamina.
• Lamin A is synthesized as a precursor and matures after integration.
• Phosphorylation is the sole modification of lamins during mitosis, with significantly higher phosphate levels during metaphase.
• Lamina depolymerization during cell division is linked to increased lamin phosphorylation.

Conclusions:

• The nuclear lamina is a polymeric assembly of lamins A, B, and C, with lamin B potentially anchoring it to the inner nuclear membrane.
• Lamins are integrated into the lamina structure post-synthesis, with differing assembly rates.
• Reversible depolymerization of the lamina during mitosis is regulated by enzymic phosphorylation of lamins, suggesting a model for nuclear envelope disassembly.