Acumentin, a protein in macrophages which caps the "pointed" end of action filaments.

Macrophages have a calcium-insensitive protein which binds to and inhibits monomer addition at the "pointed" end of actin filaments labelled with heavy meromyosin. This protein, in concert with calcium-sensitive proteins already shown to bind the "barbed" end of filaments, can lead to complete control of actin filament length in the cytoplasm. The blocking of actin filament ends in low as well as high calcium concentrations would prevent treadmilling in living cells.