Capping protein regulators fine-tune actin assembly dynamics

Abstract

Capping protein (CP) regulates actin polymerization by binding the barbed end of an actin filament, which blocks addition and loss of actin subunits. Recent structural and biochemical studies provide new insight into how cells control the actin capping activity of CP. Several molecules indirectly regulate CP by interacting with filament barbed ends and preventing binding of CP; others bind directly to CP and sterically block its interaction with an actin filament. A diverse and otherwise unrelated set of proteins contains a motif for CP regulation termed the “Capping Protein Interaction” (CPI) motif. These proteins bind directly to CP, recruit or target CP to a subcellular location, and modulate its actin-capping activity via allosteric effects.

V-1 regulates capping protein activity in vivo

“…One prediction of this model is that cells expressing a version of CP that cannot see the CPI motif in CARMIL and related proteins (e.g., CD2AP and CKIP) should phenocopy Arp2/3-inhibited cells. This prediction has recently received direct support by the work of Edwards et al (29).Studies using gene ablation or RNAi have demonstrated that CARMIL proteins play important roles in the formation of cortical actin structures like pseudopodia and macropinocytic crowns in Dictyostelium and lamellipodia in tissue culture cells (1,19). In contrast, relatively little is known regarding the importance of V-1 in vivo (30-32), especially with respect to effects on actin assembly and organization.…”

“…Studies using gene ablation or RNAi have demonstrated that CARMIL proteins play important roles in the formation of cortical actin structures like pseudopodia and macropinocytic crowns in Dictyostelium and lamellipodia in tissue culture cells (1,19). In contrast, relatively little is known regarding the importance of V-1 in vivo (30-32), especially with respect to effects on actin assembly and organization.…”

“…To date, two direct regulators of CP activity have been identified. The first, CARMIL, is a ∼125-kDa protein that binds CP tightly via a small, highly conserved domain known as CAH3 or CPI (1,19). CP bound to CPI binds the barbed end with an affinity of ∼50 nM, which, although still quite significant, is ∼100-fold weaker than free CP (20,21).…”

“…Finally, we present evidence that V-1's ability to sequester CP is regulated by phosphorylation, suggesting that cells may manipulate the level of active CP to tune their "actin phenotype." actin | capping protein | V-1 | myotrophin | CARMIL T he addition of Capping Protein (CP) to seed-initiated actin polymerization assays results in the rapid cessation of polymerization because CP binds with very high affinity to the fastgrowing barbed end of the actin filament to block further monomer addition (1). Direct extrapolation of this simple, potent biochemical property would suggest that the cell's content of F-actin should rise and fall as its content of CP is artificially forced to fall and rise, respectively.…”

Adaptor Protein CD2AP and L-type Lectin LMAN2 Regulate Exosome Cargo Protein Trafficking through the Golgi Complex

“…To circumvent this promiscuity, we focused our investigation on the early intracellular trafficking events of newly synthesized GPRC5B proteins. We show that most newly synthesized GPRC5B protein is released in exosomes as opposed to microvesicles and that an adaptor protein, CD2AP (27), mediates internalization of cell surfacelocalized GPRC5B as an early event. Surprisingly, we found that the internalized GPRC5B proteins subsequently accumulated in the Golgi complex prior to exosomal release and that the L-type lectin LMAN2 (28,29) impedes the exosomal release of GPRC5B specifically from the trans Golgi network (TGN), possibly by interfering with the physical association of GPRC5B and GGA1, an adaptor protein implicated in TGN-to-endosome trafficking (30,31), or possibly by opposing the biosynthetic secretory trafficking through TGN-to-endoplasmic reticulum (ER) retrograde transport (28).…”