SPIN90 associates with mDia1 and the Arp2/3 complex to regulate cortical actin organization.

Cell shape is controlled by the submembranous cortex, an actomyosin network mainly generated by two actin nucleators: the Arp2/3 complex and the formin mDia1. Changes in relative nucleator activity may alter cortical organization, mechanics and cell shape. Here we investigate how nucleation-promoting factors mediate interactions between nucleators. In vitro, the nucleation-promoting factor SPIN90 promotes formation of unbranched filaments by Arp2/3, a process thought to provide the initial filament for generation of dendritic networks. Paradoxically, in cells, SPIN90 appears to favour a formin-dominated cortex. Our in vitro experiments reveal that this feature stems mainly from two mechanisms: efficient recruitment of mDia1 to SPIN90-Arp2/3 nucleated filaments and formation of a ternary SPIN90-Arp2/3-mDia1 complex that greatly enhances filament nucleation. Both mechanisms yield rapidly elongating filaments with mDia1 at their barbed ends and SPIN90-Arp2/3 at their pointed ends. Thus, in networks, SPIN90 lowers branching densities and increases the proportion of long filaments elongated by mDia1.

Viral Hijacking of Formins in Neurodevelopmental Pathologies.

The 2015 Zika virus (ZIKV) outbreak caused global concern when it was determined to cause microcephaly, hearing loss, and other neurodevelopmental manifestations upon fetal exposure. Significant progress has been made in our understanding of the interactions between ZIKV and the pregnant host, but there is still a critical need to understand how ZIKV and other neurotropic viruses affect fetal neurodevelopment. Diaphanous-related formins (Diaphs) have recently been identified as microcephaly-associated proteins in humans and mice. Mutations in Diaphs affect the function of neural progenitor cells, much like prenatal viral infection. We present a novel hypothesis that viruses 'hijack' Diaphs in neural progenitor cells, causing autonomous differentiation and apoptosis of neural progenitor cells, which could potentially contribute to virus-associated neurological pathologies.

Actin motility: formin a SCAry tail.

A new biochemical analysis has revealed that the Rickettsia bacterial protein Sca2--recently shown to be essential for virulence and actin-dependent motility--assembles actin filaments using a mechanism that functionally resembles the processive elongation tactics used by formins.

WASP family members and formin proteins coordinate regulation of cell protrusions in carcinoma cells.

We examined the role of the actin nucleation promoters neural Wiskott-Aldrich syndrome protein (N-WASP) and WAVE2 in cell protrusion in response to epidermal growth factor (EGF), a key regulator in carcinoma cell invasion. We found that WAVE2 knockdown (KD) suppresses lamellipod formation and increases filopod formation, whereas N-WASP KD has no effect. However, simultaneous KD of both proteins results in the formation of large jagged protrusions with lamellar properties and increased filopod formation. This suggests that another actin nucleation activity is at work in carcinoma cells in response to EGF. A mammalian Diaphanous-related formin, mDia1, localizes at the jagged protrusions in double KD cells. Constitutively active mDia1 recapitulated the phenotype, whereas inhibition of mDia1 blocked the formation of these protrusions. Increased RhoA activity, which stimulates mDia1 nucleation, was observed in the N-WASP/WAVE2 KD cells and was shown to be required for the N-WASP/WAVE2 KD phenotype. These data show that coordinate regulation between the WASP family and mDia proteins controls the balance between lamellar and lamellipodial protrusion activity.