Structures of FHOD1-Nesprin1/2 complexes reveal alternate binding modes for the FH3 domain of formins

Abstract

The nuclear position in eukaryotic cells is controlled by a nucleo-cytoskeletal network, with important roles in cell differentiation, division and movement. Forces are transmitted through conserved linker of nucleoskeleton and cytoskeleton (LINC) complexes that traverse the nuclear envelope and engage on either side of the membrane with diverse binding partners. Nesprin-2 giant (Nes2G), a LINC element in the outer nuclear membrane, connects to the actin network directly as well as through FHOD1, a formin whose major activity is bundling actin. Much of the molecular details of this process remain poorly understood. Here, we report the crystal structure of Nes2G bound to FHOD1. We show that the G-binding domain of FHOD1 is rather a spectrin repeat binding enhancer for the neighboring FH3 domain, possibly establishing a common binding mode among this subclass of formins. The FHOD1-Nes2G complex structure suggests that spectrin repeat binding by FHOD1 is likely not regulated by the DAD helix of FHOD1. Finally, we establish that Nes1G also has one FHOD1 binding spectrin repeat, indicating that these abundant, giant Nesprins have overlapping functions in actin-bundle recruitment for nuclear movement.