RAC1 is a small 21 kDa RHO GTPase that plays a pivotal role in regulating actin cytoskeletal dynamics and cell growth. Alterations in the activity of RAC1 are implicated in a range of diseases, including cancer. Increased RAC1 activity, due to overexpression and/or activating mutations, drives transcriptional upregulation, reactive oxygen species production, mesenchymal-to-epithelial transition, membrane ruffling, and uncontrolled cell proliferation, which are hallmarks of an oncogenic phenotype. While RAC1-activating mutations alone do not appear sufficient to transform cells, their combination with other common mutations, such as BRAF, NRAS, or NF1, have been linked to drug resistance and significantly worsen patient prognosis and hinder treatment responses. The precise mechanisms underlying drug resistance, and the regulation of RAC1 splicing remain poorly understood. RAC1 is a challenging therapeutic target due to its ubiquitous presence and essential cellular functions. To date, there are no established standard treatments for cancers that harbour an additional RAC1 mutation or for RAC1-mediated drug resistance. Current experimental strategies aim to target RAC1 localization, its activators (e.g. guanine nucleotide exchange factors) and downstream effectors. Regulating RAC1 expression by targeting epigenetic regulators, and direct targeting of RAC1 itself, may also be possible in the near future.
KRAS is the most frequently mutated oncogene in human cancer. In multiple types of cancer, a missense mutation at codon 12 substitutes a glycine for a cysteine, causing hyperactivation of the GTPase and enhanced MAPK signalling. Recent drug discovery efforts culminating from work during the past decade have resulted in two FDA-approved inhibitors, sotorasib and adagrasib, which target the KRASG12C mutant allele. Ongoing medicinal chemistry efforts across academia and industry have continued developing more potent and efficacious KRASG12C inhibitors. One agent in late-stage clinical trials, divarasib, has demonstrated robust overall response rates, in some cases greater than currently approved agents. Divarasib also exhibits enhanced covalent target engagement in vitro and significant specificity for KRASG12C, yet the structural details of its binding have not been published. Here we report a high-resolution crystal structure of cysteine-light KRAS-4BG12C in complex with divarasib. Though it binds in the same allosteric pocket as sotorasib and adagrasib, the switch-II loop in each crystal structure takes on a distinct conformation differing as much as 5.6 Å between the Cα atom of residue 65 with sotorasib. Additionally, we highlight structural features of the drug complex that may guide future medicinal chemistry efforts targeting various KRAS alleles.
Morphogenesis plays a pivotal role in the infection process of Trichophyton rubrum, a primary aetiological agent of dermatophytosis that inhabits superficial human tissues. T. rubrum proliferates by extending filamentous structures, or hyphae, which are composed of highly polarized cells. In response to environmental stimuli, T. rubrum also produces asexual spores called microconidia, consisting of individual cells. Although these dynamic morphological changes are critical for T. rubrum proliferation and environmental adaptation, the molecular mechanisms underlying these processes remain poorly understood. In previous research, we demonstrated that repressing Cdc24, a guanine nucleotide exchange factor (GEF) for the small GTPases Rac and Cdc42, disrupts fungal cell polarity and impairs hyphal formation in T. rubrum. In this study, we show that Rac deficiency in the Δrac strain minimally affects hyphal formation, as indicated by the cell polarity index (the ratio of a cell's long to short diameter in hyphae). However, simultaneous Rac deficiency and Cdc42 repression in the Δrac/Pctr4 cdc42 strain significantly disrupted cell polarity, suggesting that Rac and Cdc42 perform overlapping functions in hyphal morphogenesis. Interestingly, Rac deficiency inhibited microconidia formation, whereas cdc42 repression had no detectable impact. Furthermore, adding cysteine, a radical scavenger abundant in keratins, to the growth medium reduced microconidia production in the wild-type strain but not in the Δrac strain. These findings suggest that cysteine in host tissues inhibits Rac-mediated microconidia formation. Overall, this study identifies Rac as a key regulator of T. rubrum morphogenesis, with specific roles in both hyphal development and microconidia formation.
RhoA, Rac1 and CDC42 are small G proteins that play a crucial role in regulating various cellular processes, such as the formation of actin cytoskeleton, cell shape and cell migration. Our recent results suggest that MLL is responsible for maintaining the balance of these small Rho GTPases. MLL depletion affects the stability of Rho GTPases, leading to a decrease in their protein levels and loss of activity. These changes manifest in the form of abnormal cell shape and disrupted actin cytoskeleton, resulting in reduced cell spreading and migration. Interestingly, their chaperone protein RhoGDI1 but not the Rho GTPases, is under the direct transcriptional regulation of MLL. Here, we comment on the possible implications of these observations on the signalling by Rho GTPases protein network.
Rho proteins are part of the Ras superfamily, which function to modulate cytoskeletal dynamics including cell adhesion and motility. Recently, an activating mutation in Cdc42, a Rho family GTPase, was found in a patient sample of melanoma. Previously, our work had shown the PI3K was important downstream of mutationally active Cdc42. Our present study sought to determine whether PI3K was a crucial downstream partner for Cdc42 in a melanoma cells line with a BRAF mutation, which is the most common mutation in cutaneous melanoma. In this work we were able to show that Cdc42 contributes to proliferation, anchorage-independent growth, cell motility and invasion. Treatment with a pan-PI3K inhibitor was able to effectively ameliorate all these cancer phenotypes. These data suggest that PI3K may be an important target downstream of Cdc42 in melanoma.
