Domain structure of cortactin and associated binding proteins. Specific cortactin domains are described in the text. Proteins outlined in red represent binding partners with known interaction sites. Proteins outlined in blue represent known binding partners with undetermined binding sites. Proteins outlined in green represent kinases known to phosphorylate cortactin at the indicated sites.

Cortactin-mediated activation of Arp2/3 actin nucleation and phosphorylation-independent interactions with N-WASp and WIP. A. Cortactin functions as an NPF for Arp2/3 complex. Cortactin binds the Arp2/3 through its NTA domain and F-actin through the fourth tandem repeat, resulting in direct activation Arp2/3 actin nucleation activity. B. Cortactin activates N-WASp. Cortactin enhances N-WASp mediated Arp2/3 activation by disrupting N-WASp autoinhibition through binding of the cortactin SH3 domain to the PRR region of N-WASp. The CA domain of N-WASp binds to and activates Arp2/3, while the WH2 domain binds ATP-loaded G-actin. In this scenario, N-WASp appears to be the dominant activator of Arp2/3 complex. C. WIP enhances cortactin-mediated Arp2/3 activation. The cortactin SH3 domain interacts with the WIP cortactin binding domain (CBD), presumably bringing tandem WH2 domains with bound G-actin in close proximity with Arp2/3 complex activated by the cortactin NTA, serving to enhance cortactin-mediated Arp2/3 activation.

Increased complexity and role of tyrosine phosphorylation in cortactin-mediated Arp2/3 complex activation. A. Activation of Arp2/3 complex by a cortactin/WIP/N-WASp complex. Binding of the cortactin SH3 domain to the CBD of WIP, coupled with the interaction of the WIP WASp binding domain (WBD) to the WH1 domain of N-WASp creates a trimeric complex capable of activating Arp2/3 actin nucleation activity. B. Tyrosine phosphorylation of cortactin influences Arp2/3 activation. The adaptor NCK binds cortactin through an SH2-phosphotyrosine mediated interaction. An SH3 domain of Nck1 in turn interacts with proline-rich regions on WIP, thereby positioning WIP to enhance Arp2/3 activation in an analogous manner to the positioning of WIP in Fig. 2C. C. N-WASP induced Arp2/3 activation mediated by an Nck1/cortactin complex. Nck1 binding to tyrosine phosphorylated cortactin as in B can also interact with and activate N-WASp through binding of one or more Nck1 SH3 domains binding to the proline rich region (PRR) of N-WASp, resulting in Arp2/3 activation. D. Multiprotein complexes of tyrosine phosphorylated cortactin, Nck1, N-WASp and WIP incorporating aspects of Fig 2 and 3 are conceivable, with the possible binding of WIP to the cortactin SH3 domain and/or N-WASp in addition to the Nck1/N-WASp complex described in C. In instances where N-WASp is present, cortactin appears to function as an adaptor rather than an NPF. Abbreviations: N-WASp-WH1: WASP-homology 1; GBD: GTPase-binding domain; PRR: proline-rich region; W: WASP-homology 2 domain (WH2); CA: Arp2/3 binding connector and acidic domain. WIP – CBD: cortactin binding domain; NBD: Nck binding domain; WBD: N-WASp binding domain. Nck1 – SH3: Src-homology 3 domain; SH2: Src-homology 2 domain. Cortactin – NTA: N-terminal acidic domain; Helix: alpha-helical region; P-rich: proline-rich region. For simplicity, actin filaments are shown where the Arp2/3 complex is located to demonstrate nucleation activity.

Schematic illustration of membrane- and cytoplasmic-based signaling molecules and their post-translational modifications on cortactin function. Kinase-based signaling cascades initiated by adhesion or growth factor receptors are mediated by select serine/threonine and tyrosine kinases that serve to modify cortactin at the indicated amino acids. Cortactin function is also regulated by a cycle of acetylation/deacetylation. Functional consequences of each specific modification are indicated.