Search

Platelet function at vascular injury sites is tightly regulated through the actin cytoskeleton. The Wiskott–Aldrich syndrome protein-family verprolin-homologous protein (WAVE)-regulatory complex (WRC) activates lamellipodia formation via ARP2/3, initiated by GTP-bound RAC1 interacting with the WRC subunit CYFIP1. The protein FAM49b (Family of Unknown Function 49b), also known as CYRI-B (CYFIP-Related RAC Interactor B), has been found to interact with activated RAC1, leading to the negative regulation of the WRC in mammalian cells. To investigate the role of FAM49b in platelet function, we studied platelet-specific Fam49b−/−-, Cyfip1−/−-, and Cyfip1/Fam49b−/−-mice. Platelet counts and activation of Fam49b−/− mice were comparable to those of control mice. On fully fibrinogen-coated surfaces, Fam49b−/−-platelets spread faster with an increased mean projected cell area than control platelets, whereas Cyfip1/Fam49b−/−-platelets did not form lamellipodia, phenocopying the Cyfip1−/−-platelets. However, Fam49b−/−-platelets often assumed a polarized shape and were more prone to migrate on fibrinogen-coated surfaces. On 2D structured micropatterns, however, Fam49b−/−-platelets displayed reduced spreading, whereas spreading of Cyfip1−/−- and Cyfip1/Fam49b−/−-platelets was enhanced. In summary, FAM49b contributes to the regulation of morphology and migration of spread platelets, but to exert its inhibitory effect on actin polymerization, the functional WAVE complex must be present. [ABSTRACT FROM AUTHOR]

In eukaryotes, numerous fundamental processes are controlled by the WAVE regulatory complex (WRC) that regulates cellular actin polymerization, crucial for cell motility, cell–cell adhesion and epithelial differentiation. Actin assembly is triggered by interaction of the small GTPase Rac1 with CYFIP1, a key component of the WRC. Previously known as FAM49B, CYRI‐B is a protein that is highly conserved across the Eukaryota and has recently been revealed to be a key regulator of Rac1 activity. Mutation of CYRI‐B or alteration of its expression therefore leads to altered actin nucleation dynamics, with impacts on lamellipodia formation, cell migration and infection by intracellular pathogens. In addition, knockdown of CYRI‐B expression in cancer cell lines results in accelerated cell proliferation and invasiveness. Here, the structure of Rhincodon typus (whale shark) CYRI‐B is presented, which is the first to be reported of any CYRI family member. Solved by X‐ray crystallography, the structure reveals that CYRI‐B comprises three distinct α‐helical subdomains and is highly structurally related to a conserved domain present in CYFIP proteins. The work presented here establishes a template towards a better understanding of CYRI‐B biological function. [ABSTRACT FROM AUTHOR]