Phosphorylation of caldesmon by cdc2 kinase.

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• Mitosis involves significant cellular structural reorganization.
• Nonmuscle caldesmon dissociation from microfilaments during mitosis is linked to phosphorylation.
• Smooth muscle caldesmon's role in mitosis requires further investigation.

Purpose of the Study:

• To investigate the phosphorylation of smooth muscle caldesmon by cdc2 kinase.
• To determine the effect of phosphorylation on caldesmon's interaction with F-actin and calmodulin.
• To identify the phosphorylation sites on the caldesmon molecule.

Main Methods:

• In vitro phosphorylation of smooth muscle caldesmon using cdc2 kinase from mitotic HeLa cells.
• Analysis of phosphorylation sites using tryptic mapping.
• Assessment of F-actin and calmodulin binding to phosphorylated caldesmon.

Main Results:

• Smooth muscle caldesmon was phosphorylated by cdc2 kinase, incorporating 1.2 mol of phosphate/mol of caldesmon.
• Phosphorylation occurred on both serine and threonine residues.
• F-actin or calmodulin binding blocked caldesmon phosphorylation, and phosphorylation reduced caldesmon's F-actin binding affinity.
• Phosphorylation sites were localized to a COOH-terminal 10,000-Da fragment.

Conclusions:

• Phosphorylation of smooth muscle caldesmon by cdc2 kinase significantly weakens its binding to F-actin.
• This phosphorylation event likely contributes to the disassembly of microfilament bundles during mitotic prophase.
• The findings support a regulatory role for caldesmon phosphorylation in cytoskeletal dynamics during mitosis.