Interaction of phalloidin with chemically modified actin.

Area of Science:

• Biochemistry
• Molecular Biology
• Cellular Dynamics

Background:

• Actin polymerization is essential for muscle contraction and cell motility.
• Phalloidin is a toxin that binds to actin and stabilizes its filaments.
• Specific tyrosine residues in actin may play roles in polymerization and ligand binding.

Purpose of the Study:

• To investigate the role of specific tyrosine residues (Tyr-69 and Tyr-53) in actin polymerization and phalloidin binding.
• To elucidate the mechanism by which phalloidin induces actin polymerization.

Main Methods:

• Chemical modification of actin at Tyr-69 and Tyr-53 using tetranitromethane and 5-diazonium-(1H)tetrazole, respectively.
• Assessing the polymerizability of modified actin.
• Evaluating the binding of phalloidin to modified actin.
• Measuring myosin binding and ATPase activity of modified actin filaments.

Main Results:

• Modification of Tyr-69 did not impair actin polymerization or its interaction with phalloidin and myosin.
• Modification of Tyr-53 completely blocked actin polymerization and phalloidin binding.
• Phalloidin binding to actin appears to induce a conformational change essential for polymerization.

Conclusions:

• Tyr-69 is not directly involved in actin-actin or actin-myosin binding sites.
• The binding site for phalloidin on actin is critical for polymerization and associated conformational changes.
• Phalloidin may stabilize a polymerizable conformation of actin monomer, leading to irreversible polymerization.