Cap proteins and tropomodulin

The dynamics and treadmilling of actin filaments are regulated by capping proteins that specifically bind to the ends of filaments. If this didn’t happen actin filaments would grow and shrink in an uncontrolled manner. CapZ binds with very high affinity to the plus end and inhibits both subunit addition and loss. The affinity of CapZ is regulated by lipids at the plasmamembrane - so filaments are less likely to be capped near the cell cortex and thus can grow. Another protein, tropomodulin, binds the minus ends of filaments and in doing so promotes filament growth. Tropomodulin usually works in conjunction with tropomyosin (which is a rod like molecule that binds and stabilises the filament) in muscle