The diaphanous inhibitory domain/diaphanous autoregulatory domain interaction is able to mediate heterodimerization between mDia1 and mDia2

Formins are multidomain proteins that regulate numerous cytoskeleton-dependent cellular processes. These effects are mediated by the presence of two regions of homology, formin homology 1 and FH2. The diaphanous-related formins (DRFs) are distinguished by the presence of interacting N- and C-terminal regulatory domains. The GTPase binding domain and diaphanous inhibitory domain ( DID) are found in the N terminus and bind to the diaphanous autoregulatory domain ( DAD) found in the C terminus. Adjacent to the DID is an N- terminal dimerization motif ( DD) and coiled-coil region ( CC). The N terminus of Dia1 is also proposed to contain a Rho-independent membrane-targeting motif. We undertook an extensive structure/function analysis of the mDia1 N terminus to further our understanding of its role in vivo. We show here that both DID and DD are required for efficient autoinhibition in the context of full-length mDia1 and that the DD of mDia1 and mDia2, like formin homology 2, mediates homo- but not heterodimerization with other DRF family members. In contrast, our results suggest that the DID/DAD interaction mediates heterodimerization of full-length mDia1 and mDia2 and that the auto-inhibited conformation of DRFs is oligomeric. In addition, we also show that the DD/CC region is required for the Rho-independent membrane targeting of the isolated N terminus.