The auto-inhibitory state of Rho guanine nucleotide exchange factor ARHGEF5/TIM can be relieved by targeting its SH3 domain with rationally designed peptide aptamers.

The short isoform of Rho guanine nucleotide exchange factor ARHGEF5 is known as TIM, which plays diverse roles in, for example, tumorigenesis, neuronal development and Src-induced podosome formation through the activation of its substrates, the Rho family of GTPases. The activation is auto-inhibited by a putative helix N-terminal to the DH domain of TIM, which is stabilized by the intramolecular interaction of C-terminal SH3 domain with a poly-proline sequence between the putative helix and the DH domain. In this study, we systematically investigated the structural basis, energetic landscape and biological implication underlying TIM auto-inhibition by using atomistic molecular dynamics simulations and binding free energy analysis. The computational study revealed that the binding of SH3 domain to poly-proline sequence is the prerequisite for the stabilization of TIM auto-inhibition. Thus, it is suggested that targeting SH3 domain with competitors of the poly-proline sequence would be a promising strategy to relieve the auto-inhibitory state of TIM. In this consideration, we rationally designed a number of peptide aptamers for competitively inhibiting the SH3 domain based on modeled TIM structure and computationally generated data. Peptide binding test and guanine nucleotide exchange analysis solidified that these designed peptides can both bind to the SH3 domain potently and activate TIM-catalyzed RhoA exchange reaction effectively. Interestingly, a positive correlation between the peptide affinity and induced exchange activity was observed. In addition, separate mutation of three conserved residues Pro49, Pro52 and Lys54 - they are required for peptide recognition by SH3 domain -- in a designed peptide to Ala would completely abolish the capability of this peptide activating TIM. All these come together to suggest an intrinsic relationship between peptide binding to SH3 domain and the activation of TIM.

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