Dissecting requirements for auto-inhibition of actin nucleation by the formin, mDia1

The mammalian formin, mDial, is an actin nucleation factor. Experiments in cells and in vitro show that the N-terminal region potently inhibits nucleation by the formin homology 2 (FH2) domain-containing C terminus and that RhoA binding to the N terminus partially relieves this inhibition. Cellular experiments suggest that potent inhibition depends upon the presence of the diaphanous auto-regulatory domain (DAD) C-terminal to FH2. In this study, we examine in detail the N-terminal and C-terminal regions required for this inhibition and for RhoA relief. Limited proteolysis of an N-terminal construct from residues 1-548 identifies two stable truncations: 129-548 and 129-369. Analytical ultracentrifugation suggests that 1-548 and 129-548 are dimers, whereas 129-369 is monomeric. All three N-terminal constructs inhibit nucleation by the full C terminus. Although inhibition by 1-548 is partially relieved by RhoA, inhibition by 129-548 or 129-369 is RhoA-resistant. At the C terminus, DAD deletion does not affect nucleation but decreases inhibitory potency of 1-548 by 20,000-fold. Synthetic DAD peptide binds both 1-548 and 129-548 with similar affinity and partially relieves nucleation inhibition. C-terminal constructs are stable dimers. Our conclusions are as follows: 1) DAD is an affinity-enhancing motif for auto-inhibition; 2) an N-terminal domain spanning residues 129-369 (called DID for diaphanous inhibitory domain) is sufficient for autoinhibition; 3) a dimerization region C-terminal to DID increases the inhibitory ability of DID; and 4) DID alone is not sufficient for RhoA relief of auto-inhibition, suggesting that sequences N-terminal to DID are important to RhoA binding. An additional finding is that FH2 domain-containing constructs of mDial and mDia2 lose >75% nucleation activity upon freeze-thaw.