Protein kinase activity associated with the nuclear lamina

Area of Science:

• Cell biology
• Molecular biology
• Biochemistry

Background:

• The nuclear lamina, a protein meshwork lining the inner nuclear membrane, plays crucial roles in nuclear structure and gene regulation.
• Protein phosphorylation is a key post-translational modification regulating protein function, including components of the nuclear lamina.

Purpose of the Study:

• To identify and characterize protein kinase activity associated with the nuclear lamina in Ehrlich ascites tumor cells.
• To investigate the substrates and properties of this nuclear lamina-associated kinase.

Main Methods:

• Isolation of a nuclear lamina-enriched fraction from Ehrlich ascites tumor cells.
• In vitro kinase assays using nuclear lamins and other nuclear proteins as substrates.
• Characterization of enzyme properties (thermolability, ion/cAMP dependence, inhibitor sensitivity, urea stability).
• Phosphorylation site analysis using 2D phosphopeptide mapping.
• Photoaffinity labeling to identify potential kinase candidates.

Main Results:

• A tightly bound protein kinase activity was identified in the nuclear lamina fraction.
• The kinase phosphorylates nuclear lamins, a 52 kDa protein, and other minor components.
• The enzyme is thermolabile, Ca2+/cAMP-independent, quercetin-sensitive, and inactivated by 6 M urea.
• Lamin phosphorylation occurs on serine residues, with multiple phosphorylation sites identified.
• Phosphopeptide maps show similarity to in vivo labeled lamins, and photoaffinity labeling suggests candidate kinase polypeptides.

Conclusions:

• Nuclear lamina-enriched fractions contain active protein kinase(s) that phosphorylate nuclear lamins and other nuclear components.
• The characterized kinase exhibits specific properties and phosphorylates lamins on serine residues.
• These findings contribute to understanding the regulation of nuclear lamina structure and function through phosphorylation.