Actin capping protein and its inhibitor CARMIL: how intrinsically disordered regions function

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• Actin capping protein (CP) regulates actin filament dynamics by binding to the barbed end.
• CARMIL inhibits CP's interaction with actin filaments through its C-terminal region.
• An allosteric inhibition model involving a population shift mechanism has been proposed for CARMIL's suppression of CP.

Purpose of the Study:

• To elucidate the structural basis of CARMIL-mediated inhibition of actin capping protein.
• To investigate the role of the β-tentacle and CP-binding motif in CP regulation.
• To validate the population shift mechanism in CARMIL's suppression of CP.

Main Methods:

• Crystal structure determination of CP in complex with a CARMIL-derived peptide (CA32).
• Normal mode analysis using an elastic network model on available CP/CARMIL peptide complex structures.
• Analysis of the intrinsically disordered nature of the β-tentacle and CP-binding motif.

Main Results:

• The crystal structure revealed an α-helical form of the β-tentacle in the CP/CA32 complex.
• Normal mode analysis indicated the CP-binding motif is necessary and sufficient to alter CP fluctuations.
• These alterations in CP fluctuation are crucial for attenuating barbed-end-capping activity.

Conclusions:

• The CP-binding motif of CARMIL is key to modulating CP's dynamic fluctuations.
• The population shift mechanism explains how CARMIL suppresses CP's capping activity.
• The intrinsically disordered nature of the β-tentacle and CP-binding motif is central to their regulatory functions.