Capping protein regulators fine-tune actin assembly dynamics

Capping protein (CP) is a major regulator of actin assembly dynamics via the capping of actin filament barbed ends. The capping activity of CP can be regulated by a number of different proteins and phospholipids in various ways, some direct and others indirect.The capping protein interacting (CPI) motif is a 30-amino acid region necessary and sufficient to bind and inhibit CP. This motif is found in a set of unrelated proteins, many of which are involved in membrane interactions.CARMIL (capping protein, ARP2/3 and myosin I linker) family proteins contain a CPI motif, and they also contain a separate CARMIL-specific interacting (CSI) motif. In CARMIL, the CPI motif is necessary for distinct cellular functions, such as macropinocytosis.The CPI and CSI motifs are unstructured in the unbound state, but they adopt a specific structure when they bind to CP, applying themselves to the surface of CP. The CPI and CSI motifs decrease the actin capping activity of CP via an allosteric mechanism.The complex of a CPI motif-containing protein with CP retains a low level of capping activity, which raises the possibility that CPI motif-containing proteins may target CP to certain cellular locations, in addition to, or as an alternative to, simply decreasing the capping activity.Vertebrates have three distinct conserved CARMIL genes, which seem to have distinct functions in cells. Of note, CARMIL2 localizes with vimentin filaments, representing a potential novel link between the actin and intermediate filament cytoskeleton systems.