ИСТИНА

The formin Bnr1 is recruited to the bud neck in S. cerevisiae to assemble actin cables and enable cargo transport between the mother cell and the growing daughter cell. This process is regulated by Hof1, which directly binds to Bnr1 and inhibits its actin nucleation activity. In the absence of Hof1 the actin network structure becomes disorganized, and vesicle traffic is impaired; however, the details of Hof1-Bnr1 interactions have remained unclear. Here, using single particle electron microscopy, we reveal the structures of the FH2 domains of Bnr1 in complex with full-length Hof1 or with its F-BAR domain. The Hof1-Bnr1 complex is a dumbbell-shaped structure, in which the tips of an elongated F-BAR dimer hold two FH2 dimers apart, and these interactions appear to obstruct the actin-binding surfaces on the FH2. In support of this view, bulk actin assembly assays and TIRF microscopy assays show that the F-BAR domain of Hof1 directly inhibits Bnr1-mediated actin nucleation. Taken together, our results allow us to describe a mechanism for how Hof1 controls Bnr1 activity. The authors acknowledge funding from the Russian Science Foundation (grant #14-14-00234 to O.S.), the grant of the President of the Russian Federation (grant MK-2614.2018.4 to T.S.-K.), and the National Institutes of Health (RO1 GM083137 to B.G.).