The IQ motif-containing GTPase-activating protein (IQGAP) family composes of three highly-related and evolutionarily conserved paralogs (IQGAP1, IQGAP2 and IQGAP3), which fine tune as scaffolding proteins numerous fundamental cellular processes. IQGAP1 is described as an effector of CDC42, although its effector function yet re-mains unclear. Biophysical, biochemical and molecular dynamic simulation studies have proposed that IQGAP RASGAP-related domains (GRDs) bind to the switch regions and the insert helix of CDC42 in a GTP-dependent manner. Our kinetic and equilibrium studies have shown that IQGAP1 GRD binds, in contrast to its C-terminal 794 amino acids (called C794), CDC42 in a nucleotide-independent manner indicating a binding outside the switch regions. To resolve this discrepancy and move beyond the one-sided view of GRD, we carried out affinity measurements and a systematic mutational analysis of the interfacing residues between GRD and CDC42 based on the crystal structure of the IQGAP2 GRD-CDC42^Q61L GTP complex. We determined a 100-fold lower affinity of the GRD1 of IQGAP1 and of GRD2 of IQGAP2 for CDC42 mGppNHp in comparison to C794/C795 proteins. Moreover, partial and major mutation of CDC42 switch regions substantially affected C794/C795 binding but only a little GRD1 and remarkably not at all the GRD2 binding. However, we clearly showed that GRD2 contributes to the overall affinity of C795 by using a 11 amino acid mutated GRD variant. Furthermore, the GRD1 binding to the CDC42 was abolished using specific point mutations within the insert helix of CDC42 clearly supporting the notion that CDC42 binding site(s) of IQGAP GRD lies outside the switch regions among others in the insert helix. Collectively, this study provides further evidence for a mechanistic framework model that is based on a multi-step binding process, in which IQGAP GRD might act as a ‘scaffolding domain’ by binding CDC42 irrespective of its nucleotide-bound forms, followed by other IQGAP domains downstream of GRD that act as an effector domain and is in charge for a GTP-dependent interaction with CDC42.

The RTK-RAS-MAPK axis is one of the most extensively studied signaling cascades and is related to the development of both cancers and RASopathies. In the last 30 years, many ideas and approaches have emerged for directly targeting constituent members of this cascade, predominantly in the context of cancer treatment. These approaches are still insufficient due to undesirable drug toxicity, resistance, and low efficacy. Significant advances have been made in understanding the spatiotemporal features of the constituent members of the RTK-RAS-MAPK axis, which are linked and modulated by many accessory proteins. Given that the majority of such modulators are now emerging as attractive therapeutic targets, a very small number of accessory inhibitors have yet to be discovered.

Chronic liver diseases are marked by persistent inflammation and can evolve into liver fibrosis, cirrhosis, and hepatocellular carcinoma. In an affected liver, hepatic stellate cells (HSCs) transition from a quiescent to an activated state and adopt a myofibroblast-like cell phenotype. While these activated cells play a role in supporting liver regeneration, they can also have detrimental effects on liver function as the disease progresses to fibrosis and cirrhosis. These findings highlight the dynamic switching between different signaling pathways involving Ras, Rho GTPases, and Notch signaling. Notably, two specific members of the Ras and Rho GTPases, Eras and Rnd3, are predominantly expressed in quiescent HSCs, while Mras and Rhoc are more abundant in their activated forms. In addition, this study highlights the critical role of cytosolic Notch1 in quiescent HSCs and Rock in activated HSCs. We hypothesize that distinct yet interdependent intracellular signaling networks regulate HSC fate decisions in two key ways: by maintaining HSC quiescence and homeostasis and by facilitating HSC activation, thereby influencing processes such as proliferation, transdifferentiation, and mesenchymal transition. The proposed signaling model, combined with specific methodological tools for maintaining HSCs in a quiescent state, will deepen our understanding of the mechanisms underlying chronic liver disease and may also pave the way for innovative therapies. These therapies could include small molecule drugs targeting Ras- and Rho-dependent pathways.

Embryonic stem cell-expressed Ras (ERas) is an atypical constitutively active member of the Ras family and controls distinct signaling pathways, which are critical, for instance, for the maintenance of quiescent hepatic stellate cells (HSCs). Unlike classical Ras paralogs, ERas has a unique N-terminal extension (Nex) with as yet unknown function. In this study, we employed affinity pull-down and quantitative liquid chromatography-tandem mass spectrometry (LC–MS/MS) analyses and identified 76 novel binding proteins for human and rat ERas Nex peptides, localized in different subcellular compartments and involved in various cellular processes. One of the identified Nex-binding proteins is the nonmitochondrial, cytosolic arginase 1 (ARG1), a key enzyme of the urea cycle and involved in the de novo synthesis of polyamines, such as spermidine and spermine. Here, we show, for the first time, a high-affinity interaction between ERas Nex and purified ARG1 as well as their subcellular colocalization. The inhibition of ARG1 activity strikingly accelerates the activation of HSCs ex vivo, suggesting a central role of ARG1 activity in the maintenance of HSC quiescence.

Nanobodies are small, single-domain antibody fragments derived from heavy chain - only antibodies. They combine high binding affinity with advantages such as compact size, stability, solubility, and flexible epitope recognition, making them attractive tools in molecular biology and therapeutic applications. In this study, we engineered and optimized nanobodies for controlled activation of synthetic cytokine receptors, aiming to expand options for receptor customization. Specifically, we used nanobodies as extracellular domains of the gp130 receptor to induce dimerization upon antigen binding. To enable receptor activity, we introduced framework mutations that promote the formation of an i-shaped nanobody (iBody) dimer, adapted from i-shaped antibodies. These mutations enhanced dimerization and enabled low-level ligand-independent receptor activation. AlphaFold modeling identified the key amino acids responsible for forming the iBody interface. Additional modifications reduced intermolecular affinity, thereby minimizing background activation while preserving the structural features necessary for ligand-induced stimulation. This approach effectively broadened the receptor's activation range. Importantly, these framework mutations were not limited to the gp130-specific nanobody GP11 but were also functional in AIP3, an anti-idiotypic nanobody targeting palivizumab. Here, the modified nanobody fusion receptor could be activated by palivizumab, overcoming prior steric hindrance.

Both RIT1 and RIT2 are members of the RAS superfamily of small GTPases, which regulate various cellular processes. RIT1 is widely expressed, whereas RIT2 is primarily found in neuronal tissues. Dysregulation of these proteins has been associated with several human diseases, including Noonan syndrome, cancer, Parkinson’s disease, autism, and schizophrenia. Although RIT1 and RIT2 are often compared to classical RAS proteins, they exhibit distinct regulatory and biochemical properties. Here, we demonstrate that RIT1 differs from classical RAS in GTPase cycling. Unlike classical RAS proteins, RIT1 did not respond to SOS1-mediated nucleotide exchange or p120GAP-stimulated GTP hydrolysis under cell-free conditions. These results imply that RIT1 may depend on regulatory mechanisms that differ from those of classical RAS proteins. However, the relevant physiological regulators remain unknown. Disease-associated RIT1 mutations cluster around the P-loop and Switch II regions. In this transient overexpression screening system, however, these mutations had only a modest effect on the canonical MAPK, PI3K/AKT, and JNK signaling pathways in HEK293T overexpression experiments. This suggests the existence of additional context-specific effectors and regulatory factors. We demonstrate that RIT1 and RIT2 interact with membrane lipids via a basic C-terminal extension. The KRLK-containing region contributes to the binding of phosphatidylserine and phosphoinositides. Charge-reversal mutations disrupt lipid interactions and liposome binding, supporting the functional importance of this region. In a reconstituted liposome system, galectin-3 and LZTR1, but not galectin-1, reduced the interaction of GDP-loaded RIT1 and RIT2 with liposomes. These results suggest that accessory proteins may influence RIT membrane interactions. However, their cellular relevance requires further validation. Together, our findings provide biochemical insights into RIT GTPase regulation and its interactions with membrane lipids under cell-free conditions.