Interaction of nonpolymerizable actins with myosin

Area of Science:

• Biochemistry
• Molecular Biology
• Muscle Contraction

Background:

• Actin polymerization is crucial for muscle function.
• Myosin heads interact with actin filaments to generate force.
• Understanding actin-myosin interactions is key to muscle physiology.

Purpose of the Study:

• To investigate the binding properties of chemically modified actins to myosin heads.
• To elucidate the number and nature of actin-binding sites on myosin heads.
• To determine the influence of ATP and native F-actin on these interactions.

Main Methods:

• Chemical modification of G-actin with m-maleimidobenzoic acid N-hydroxysuccinimide ester (MBS) to create m-actin.
• Chemical crosslinking of F-actin with N,N'-p-phenylenedimaleimide, followed by MBS treatment, to create d-actin.
• Measurement of actin-myosin binding affinities using dissociation constants and complex stoichiometry.

Main Results:

• m-actin and d-actin retained binding ability to myosin heads, with d-actin showing higher affinity.
• d-Actin formed a 1:1 complex, while m-actin formed a 2:1 complex, indicating multiple binding sites on myosin.
• Native F-actin inhibited m- and d-actin binding, suggesting complex regulation of binding sites.

Conclusions:

• Myosin heads possess at least three distinct actin-binding sites.
• One site binds F-, m-, and d-actins; a second binds F- and m-actins; a third binds F-actin, especially with ATP.
• F-actin binding to the third site may allosterically regulate the other two sites.