Capping protein regulators fine-tune actin assembly dynamics

Abstract

Capping protein (CP) binds the fast growing barbed end of the actin filament and regulates actin assembly by blocking the addition and loss of actin subunits. Recent studies provide new insights into how CP and barbed-end capping are regulated. Filament elongation factors, such as formins and ENA/VASP (enabled/vasodilator-stimulated phosphoprotein), indirectly regulate CP by competing with CP for binding to the barbed end, whereas other molecules, including V-1 and phospholipids, directly bind to CP and sterically block its interaction with the filament. In addition, a diverse and unrelated group of proteins interact with CP through a conserved 'capping protein interaction' (CPI) motif. These proteins, including CARMIL (capping protein, ARP2/3 and myosin I linker), CD2AP (CD2-associated protein) and the WASH (WASP and SCAR homologue) complex subunit FAM21, recruit CP to specific subcellular locations and modulate its actin-capping activity via allosteric effects.

Figure 1. Capping protein regulates actin filament dynamics during several cellular processes

As predicted by the dendritic nucleation model, CP regulates Arp2/3 complex-dependent actin assembly at various cellular membranes, including lamellipodial protrusions, at sites of endocytosis and pinocytosis and associated with endosomal compartments undergoing fission and fusion. Capping protein also regulates assembly of actin filaments of filopodia, which can arise from dendritic actin networks.

Figure 2. Modes of CP Inhibition

A) No inhibition of barbed-end capping. CP binds to the barbed end of an actin filament via electrostatic interactions at Site 1 (basic patch) and hydrophobic interactions with the beta tentacle (Site 2). Capping protein is depicted as a heterodimer composed of an alpha (green) and beta (light blue) subunit; an actin filament barbed end is depicted in blue. B) Competition with elongation factors. Formins and Ena/VASP proteins compete with CP for binding to the barbed end of an actin filament. C) Steric inhibition. V-1 / myotrophin interacts with CP on its actin-binding surface (Site 1), sterically blocking the interaction of CP with the barbed end. D) Allosteric Inhibition. CPI and / or CSI motifs bind CP in the stalk region of the CP heterodimer and induce a conformational change that promotes dissociation of CP from the barbed end. The capping protein structure is depicted as a heterodimer composed of an alpha (green) and beta (light blue) subunit.

Figure 3. CPI-motif proteins

A) Model of CARMIL domain organization and structure. The crystal structure of CARMIL1 residues 1-668 is shown as a dimer, mediated by the HD domain (light purple). Individual domains are labeled and colored as in panel B. B) common and diverse motifs of the CPI family of proteins. The CPI motif common to this family is highlighted in the dashed box (magenta). Common oligomerization domains are shaded light purple; PXXP motifs, cyan; PH domains, marine. Other domains are indicated. Binding partners for each CPI protein are indicated at downward arrows oriented toward the domain with which each interacts. Regulatory interactions (dimerization and autoinhibition) are indicated by upward arrows. Amino acid residue numbering corresponds to human isoforms.

Figure 4. Cellular functions of CPI Motif Proteins

Diverse actin-dependent processes are regulated by CPI-motif proteins. CPI-motif proteins target CP to different cellular membranes and modulate actin assembly. CARMIL and CKIP-1 directly bind the plasma membrane via interactions of a PH domain. CARMIL may also target to membranes via interactions with some myosin-I isoforms. CD2AP is enriched at the slit-diaphragm in kidney podocytes, where it interacts with cortactin, nephrin, and podocin and helps to maintain slit diaphragm integrity. Fam21 is a component of the WASH regulatory complex associated with sorting and recycling endosomes. The CP-binding activity of CAPZIP can be modulated by phosphorylation by MAP kinases.