New mechanisms and functions of actin nucleation

Abstract

In cells the de novo nucleation of actin filaments from monomers requires actin-nucleating proteins. These fall into three main families--the Arp2/3 complex and its nucleation promoting factors (NPFs), formins, and tandem-monomer-binding nucleators. In this review, we highlight recent advances in understanding the molecular mechanism of actin nucleation by both well-characterized and newly identified nucleators, and explore current insights into their cellular functions in membrane trafficking, cell migration and division. The mechanisms and functions of actin nucleators are proving to be more complex than previously considered, with extensive cooperation and overlap in their cellular roles.

Figure 1. Models of actin nucleation

Left: Arp2/ 3 complex is activated by binding to the CA region of NPFs and to the side of actin filaments. In turn, NPFs bind actin monomers via their WH2 domains and profilin-actin monomers via their proline-rich regions, and deliver these to a nucleating complex. NPF dimerization enhances their activity, suggesting that dimers may bind to two sites on the Arp2/ 3 complex. After branch formation, cofilin and GMF stimulate debranching by binding to F-actin and Arp2/ 3 complex, respectively. Coronin binds both to F-actin and Arp2/ 3 complex, replaces Arp2/ 3 complex, and synergizes with cofilin to promote debranching. Middle: Tandem-monomer-binding nucleators bring together actin monomers through their clustered actin-binding motifs to form a nucleus. Spire and JMY stabilize actin monomers aligned along the long-pitch helix with their WH2 domains and monomer-binding linkers (MBL). Cordon-bleu, leiomodin and dimeric APC, with their combination of WH2 domains, leucine rich repeats (LRR), tropomyosin and actin-binding helices (Tmh/ Ah), and actin-nucleating sequences (ANS1–2), stabilize cross-filament interactions along the short-pitch helix of an actin filament. Right: Formins generate actin polymerization nuclei by stabilizing actin dimers through their homodimeric FH2 domains. The FH2 dimer stays processively attached to the barbed end of an actin filament as the flanking FH1 domains deliver profilin-actin to the barbed end for continued elongation. In yeast, Bud14p interacts with the FH2 domain and displaces formins from growing barbed ends. Dotted arrows point to the cross-talk between different actin nucleators.

Figure 2. Models of the cellular localization and function of actin nucleators

(A) A depiction of the role of actin nucleators in membrane-trafficking events including endocytic internalization as well as various stages of endocytic and ER-to-Golgi trafficking. Abbreviations: EE, early endosomes; ECV/ MVB, endosomal carrier vesicles/ multivesicular bodies; LE, late endosomes; RE, recycling endosomes; Lys, lysosome; ER, Endoplasmic reticulum; ERGIC, endoplasmic reticulum-Golgi intermediate compartment. (B) Diagram of the role of actin nucleators in lamellipodia and filopodia. In the dendritic organization model of actin organization in lamellipodia, branched actin networks are nucleated by the Arp2/ 3 complex and the NPFs WAVE1/ 2 and JMY. In the linear organization model, filaments are nucleated by the Arp2/ 3 complex but are unbranched, or are nucleated by mDia2 or perhaps JMY. In the tip nucleation model of filopodium formation, bundled arrays of actin filaments are nucleated by mDia2, whereas in the convergent elongation model, filaments are nucleated by the Arp2/ 3 complex and WAVE1/ 2, and are elongated by mDia2. (C) Cartoons depicting the role of actin nucleators in cell division. During cytokinesis, formins (Cdc12p in yeast, mDia2 in mammalian cells) nucleate actin filaments from multiple nodes at the division site that then coalesce into the contractile ring in the search, capture, pull and release model. During centrosome separation, dynamic actin reorganization by Dia and Arp2/ 3 drives centrosome separation in the early syncytial Drosophila embryo. For asymmetric spindle positioning in mouse oocytes, FMN2 nucleates a dynamic actin network that moves the spindle to the cell cortex. During the segregation of protein aggregates in S. cerevisiae, Bni1p generates actin cables extending from the polarisome that are required for transport of protein aggregates from the daughter to the mother cell. In (A), (B), and (C) nucleators are color-coded as follows: Arp2/ 3 complex and NPFs (blue), formins (green), tandem-monomer-binding nucleators (yellow). Question marks indicate that the precise role of the nucleating protein is unclear.