DOCK (dedicator of cytokinesis) proteins are multidomain guanine nucleotide exchange factors (GEFs) for RHO GTPases that regulate intracellular actin dynamics. DOCK proteins share catalytic (DOCK DHR2 ) and membrane-associated (DOCK DHR1 ) domains. The structurally-related DOCK1 and DOCK2 GEFs are specific for RAC, and require ELMO (engulfment and cell motility) proteins for function. The N-terminal RAS-binding domain (RBD) of ELMO (ELMO RBD ) interacts with RHOG to modulate DOCK1/2 activity. Here, we determine the cryo-EM structures of DOCK2−ELMO1 alone, and as a ternary complex with RAC1, together with the crystal structure of a RHOG−ELMO2 RBD complex. The binary DOCK2−ELMO1 complex adopts a closed, auto-inhibited conformation. Relief of auto-inhibition to an active, open state, due to a conformational change of the ELMO1 subunit, exposes binding sites for RAC1 on DOCK2 DHR2 , and RHOG and BAI GPCRs on ELMO1. Our structure explains how up-stream effectors, including DOCK2 and ELMO1 phosphorylation, destabilise the auto-inhibited state to promote an active GEF.

In higher eukaryotes, one of the two arginyl-tRNA synthetases (ArgRSs) has evolved to have an extended N-terminal domain that plays a crucial role in protein synthesis and cell growth and in integration into the multisynthetase complex (MSC). Here, we report a crystal structure of the MSC subcomplex comprising ArgRS, glutaminyl-tRNA synthetase (GlnRS), and the auxiliary factor aminoacyl tRNA synthetase complex-interacting multifunctional protein 1 (AIMP1)/p43. In this complex, the N-terminal domain of ArgRS forms a long coiled-coil structure with the N-terminal helix of AIMP1 and anchors the C-terminal core of GlnRS, thereby playing a central role in assembly of the three components. Mutation of AIMP1 destabilized the N-terminal helix of ArgRS and abrogated its catalytic activity. Mutation of the N-terminal helix of ArgRS liberated GlnRS, which is known to control cell death. This ternary complex was further anchored to AIMP2/p38 through interaction with AIMP1. These findings demonstrate the importance of interactions between the N-terminal domains of ArgRS and AIMP1 for the catalytic and noncatalytic activities of ArgRS and for the assembly of the higher-order MSC protein complex.arginyl-tRNA synthetase | multisynthetase complex | crystal structure | AIMP1 | glutaminyl-tRNA synthetase

Edited by Michael Ibba Keywords:Arginyl-tRNA synthetase a b s t r a c t Arginyl-tRNA synthetase (ArgRS) is a tRNA-binding protein that catalyzes the esterification of L-arginine to its cognate tRNA. L-Canavanine, a structural analog of L-arginine, has recently been studied as an anticancer agent. Here, we determined the crystal structures of the apo, L-arginine-complexed, and L-canavanine-complexed forms of the cytoplasmic free isoform of human ArgRS (hArgRS). Similar interactions were formed upon binding to L-canavanine or L-arginine, but the interaction between Tyr312 and the oxygen of the oxyguanidino group was a little bit different. Detailed conformational changes that occur upon substrate binding were explained. The hArgRS structure was also compared with previously reported homologue structures. The results presented here may provide a basis for the design of new anticancer drugs, such as L-canavanine analogs.

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