A fluorescent probe for conformational changes in skeletal muscle G-actin

Area of Science:

• Biochemistry
• Molecular Biology
• Muscle Physiology

Background:

• Actin is a crucial protein in muscle contraction and cellular structure.
• Understanding actin's conformational changes is key to muscle function.
• Fluorescent labeling provides a sensitive method to study protein dynamics.

Purpose of the Study:

• To investigate the conformational changes of G-actin in response to divalent metal ions (Mg2+, Ca2+) and KCl.
• To quantify the binding affinities and competitive interactions between Mg2+ and Ca2+ with G-actin.
• To explore the relationship between G-actin fluorescence, conformational state, and functional activity.

Main Methods:

• Rabbit skeletal muscle actin was labeled with N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine (1,5-I-AEDANS).
• Fluorescence spectroscopy was used to monitor changes in labeled G-actin.
• Divalent metal ions (Mg2+, Ca2+) and KCl were titrated to observe fluorescence enhancement and quenching.

Main Results:

• Mg2+ and KCl induced rapid fluorescence enhancement in labeled G-actin.
• Mg2+ and Ca2+ competed for binding sites on G-actin, with dissociation constants of 35 microM for Mg2+ and 10 microM for Ca2+.
• Ca2+ reversed the Mg2+-induced fluorescence enhancement, indicating a reversible conformational change.
• KCl-induced fluorescence enhancement was rapid and distinct from metal-ion-induced changes.
• Loss of fluorescence correlated with G-actin inactivation.

Conclusions:

• Divalent metal ions induce reversible conformational changes in G-actin, affecting its fluorescence.
• Mg2+ and Ca2+ exhibit competitive binding to G-actin.
• KCl induces a rapid, potentially different, conformational change in G-actin.
• Fluorescence changes in labeled G-actin are linked to its functional state.