Cells respond to mechanical stress by rapid disassembly of caveolae.

The functions of caveolae, the characteristic plasma membrane invaginations, remain debated. Their abundance in cells experiencing mechanical stress led us to investigate their role in membrane-mediated mechanical response. Acute mechanical stress induced by osmotic swelling or by uniaxial stretching results in a rapid disappearance of caveolae, in a reduced caveolin/Cavin1 interaction, and in an increase of free caveolins at the plasma membrane. Tether-pulling force measurements in cells and in plasma membrane spheres demonstrate that caveola flattening and disassembly is the primary actin- and ATP-independent cell response that buffers membrane tension surges during mechanical stress. Conversely, stress release leads to complete caveola reassembly in an actin- and ATP-dependent process. The absence of a functional caveola reservoir in myotubes from muscular dystrophic patients enhanced membrane fragility under mechanical stress. Our findings support a new role for caveolae as a physiological membrane reservoir that quickly accommodates sudden and acute mechanical stresses.

Eliminating oncogenic RAS: back to the future at the drawing board.

RAS drug development has made enormous strides in the past ten years, with the first direct KRAS inhibitor being approved in 2021. However, despite the clinical success of covalent KRAS-G12C inhibitors, we are immediately confronted with resistances as commonly found with targeted drugs. Previously believed to be undruggable due to its lack of obvious druggable pockets, a couple of new approaches to hit this much feared oncogene have now been carved out. We here concisely review these approaches to directly target four druggable sites of RAS from various angles. Our analysis focuses on the lessons learnt during the development of allele-specific covalent and non-covalent RAS inhibitors, the potential of macromolecular binders to facilitate the discovery and validation of targetable sites on RAS and finally an outlook on a future that may engage more small molecule binders to become drugs. We foresee that the latter could happen mainly in two ways: First, non-covalent small molecule inhibitors may be derived from the development of covalent binders. Second, reversible small molecule binders could be utilized for novel targeting modalities, such as degraders of RAS. Provided that degraders eliminate RAS by recruiting differentially expressed E3-ligases, this approach could enable unprecedented tissue- or developmental stage-specific destruction of RAS with potential advantages for on-target toxicity. We conclude that novel creative ideas continue to be important to exterminate RAS in cancer and other RAS pathway-driven diseases, such as RASopathies.

Targeting prohibitins at the cell surface prevents Th17-mediated autoimmunity.

T helper (Th)17 cells represent a unique subset of CD4+ T cells and are vital for clearance of extracellular pathogens including bacteria and fungi. However, Th17 cells are also involved in orchestrating autoimmunity. By employing quantitative surface proteomics, we found that the evolutionarily conserved prohibitins (PHB1/2) are highly expressed on the surface of both murine and human Th17 cells. Increased expression of PHBs at the cell surface contributed to enhanced CRAF/MAPK activation in Th17 cells. Targeting surface-expressed PHBs on Th17 cells with ligands such as Vi polysaccharide (Typhim vaccine) inhibited CRAF-MAPK pathway, reduced interleukin (IL)-17 expression and ameliorated disease pathology with an increase in FOXP3+-expressing Tregs in an animal model for multiple sclerosis (MS). Interestingly, we detected a CD4+ T cell population with high PHB1 surface expression in blood samples from MS patients in comparison with age- and sex-matched healthy subjects. Our observations suggest a pivotal role for the PHB-CRAF-MAPK signalling axis in regulating the polarization and pathogenicity of Th17 cells and unveil druggable targets in autoimmune disorders such as MS.

A subset of flavaglines inhibits KRAS nanoclustering and activation.

The RAS oncogenes are frequently mutated in human cancers and among the three isoforms (KRAS, HRAS and NRAS), KRAS is the most frequently mutated oncogene. Here, we demonstrate that a subset of flavaglines, a class of natural anti-tumour drugs and chemical ligands of prohibitins, inhibit RAS GTP loading and oncogene activation in cells at nanomolar concentrations. Treatment with rocaglamide, the first discovered flavagline, inhibited the nanoclustering of KRAS, but not HRAS and NRAS, at specific phospholipid-enriched plasma membrane domains. We further demonstrate that plasma membrane-associated prohibitins directly interact with KRAS, phosphatidylserine and phosphatidic acid, and these interactions are disrupted by rocaglamide but not by the structurally related flavagline FL1. Depletion of prohibitin-1 phenocopied the rocaglamide-mediated effects on KRAS activation and stability. We also demonstrate that flavaglines inhibit the oncogenic growth of KRAS-mutated cells and that treatment with rocaglamide reduces non-small-cell lung carcinoma (NSCLC) tumour nodules in autochthonous KRAS-driven mouse models without severe side effects. Our data suggest that it will be promising to further develop flavagline derivatives as specific KRAS inhibitors for clinical applications.

Promotion of cancer cell stemness by Ras.

Cancer stem cells (CSC) may be the most relevant and elusive cancer cell population, as they have the exquisite ability to seed new tumors. It is plausible, that highly mutated cancer genes, such as KRAS, are functionally associated with processes contributing to the emergence of stemness traits. In this review, we will summarize the evidence for a stemness driving activity of oncogenic Ras. This activity appears to differ by Ras isoform, with the highly mutated KRAS having a particularly profound impact. Next to established stemness pathways such as Wnt and Hedgehog (Hh), the precise, cell cycle dependent orchestration of the MAPK-pathway appears to relay Ras activation in this context. We will examine how non-canonical activities of K-Ras4B (hereafter K-Ras) could be enabled by its trafficking chaperones calmodulin and PDE6D/PDEδ. Both dynamically localize to the cellular machinery that is intimately linked to cell fate decisions, such as the primary cilium and the centrosome. Thus, it can be speculated that oncogenic K-Ras disrupts fundamental polarized signaling and asymmetric apportioning processes that are necessary during cell differentiation.

Pharmacophore Model for SARS-CoV-2 3CLpro Small-Molecule Inhibitors and in Vitro Experimental Validation of Computationally Screened Inhibitors.

Among the biomedical efforts in response to the current coronavirus (COVID-19) pandemic, pharmacological strategies to reduce viral load in patients with severe forms of the disease are being studied intensively. One of the main drug target proteins proposed so far is the SARS-CoV-2 viral protease 3CLpro (also called Mpro), an essential component for viral replication. Ongoing ligand- and receptor-based computational screening efforts would be facilitated by an improved understanding of the electrostatic, hydrophobic, and steric features that characterize small-molecule inhibitors binding stably to 3CLpro and by an extended collection of known binders. Here, we present combined virtual screening, molecular dynamics (MD) simulation, machine learning, and in vitro experimental validation analyses, which have led to the identification of small-molecule inhibitors of 3CLpro with micromolar activity and to a pharmacophore model that describes functional chemical groups associated with the molecular recognition of ligands by the 3CLpro binding pocket. Experimentally validated inhibitors using a ligand activity assay include natural compounds with the available prior knowledge on safety and bioavailability properties, such as the natural compound rottlerin (IC50 = 37 µM) and synthetic compounds previously not characterized (e.g., compound CID 46897844, IC50 = 31 µM). In combination with the developed pharmacophore model, these and other confirmed 3CLpro inhibitors may provide a basis for further similarity-based screening in independent compound databases and structural design optimization efforts to identify 3CLpro ligands with improved potency and selectivity. Overall, this study suggests that the integration of virtual screening, MD simulations, and machine learning can facilitate 3CLpro-targeted small-molecule screening investigations. Different receptor-, ligand-, and machine learning-based screening strategies provided complementary information, helping to increase the number and diversity of the identified active compounds. Finally, the resulting pharmacophore model and experimentally validated small-molecule inhibitors for 3CLpro provide resources to support follow-up computational screening efforts for this drug target.

High-throughput amenable fluorescence-assays to screen for calmodulin-inhibitors.

The KRAS gene is highly mutated in human cancers and the focus of current Ras drug development efforts. Recently the interface between the C-terminus of K-Ras and calmodulin (CaM) was proposed as a target site to block K-Ras driven cancer cell stemness. We therefore aimed at developing a high-throughput amenable screening assay to identify novel CaM-inhibitors as potential K-Ras stemness-signaling disruptors. A modulated time-resolved Förster resonance energy transfer (mTR-FRET)-assay was developed and benchmarked against an identically designed fluorescence anisotropy (FA)-assay. In both assays, two CaM-binding peptides were labeled with Eu(III)-chelate or fluorescein and used as single-label reporter probes that were displaced from CaM upon competitor binding. Thus, peptidic and small molecule competitors with nanomolar to micromolar affinities to CaM could be detected, including a peptide that was derived from the C-terminus of K-Ras. In order to detect CaM-residue specific covalent inhibitors, a cell lysate-based Förster resonance energy transfer (FRET)-assay was furthermore established. This assay enabled us to measure the slow, residue-specific, covalent inhibition by ophiobolin A in the presence of other endogenous proteins. In conclusion, we have developed a panel of fluorescence-assays that allows identification of conventional and covalent CaM-inhibitors as potential disruptors of K-Ras driven cancer cell stemness.

Phenotypic Screening Identifies Protein Synthesis Inhibitors as H-Ras-Nanocluster-Increasing Tumor Growth Inducers.

Ras isoforms H-, N-, and K-ras are each mutated in specific cancer types at varying frequencies and have different activities in cell fate control. On the plasma membrane, Ras proteins are laterally segregated into isoform-specific nanoscale signaling hubs, termed nanoclusters. As Ras nanoclusters are required for Ras signaling, chemical modulators of nanoclusters represent ideal candidates for the specific modulation of Ras activity in cancer drug development. We therefore conducted a chemical screen with commercial and in-house natural product libraries using a cell-based H-ras-nanoclustering FRET assay. Next to established Ras inhibitors, such as a statin and farnesyl-transferase inhibitor, we surprisingly identified five protein synthesis inhibitors as positive regulators. Using commonly employed cycloheximide as a representative compound, we show that protein synthesis inhibition increased nanoclustering and effector recruitment specifically of active H-ras but not of K-ras. Consistent with these data, cycloheximide treatment activated both Erk and Akt kinases and specifically promoted H-rasG12V-induced, but not K-rasG12V-induced, PC12 cell differentiation. Intriguingly, cycloheximide increased the number of mammospheres, which are enriched for cancer stem cells. Depletion of H-ras in combination with cycloheximide significantly reduced mammosphere formation, suggesting an exquisite synthetic lethality. The potential of cycloheximide to promote tumor cell growth was also reflected in its ability to increase breast cancer cell tumors grown in ovo. These results illustrate the possibility of identifying Ras-isoform-specific modulators using nanocluster-directed screening. They also suggest an unexpected feedback from protein synthesis inhibition to Ras signaling, which might present a vulnerability in certain tumor cell types.

The efficacy of Raf kinase recruitment to the GTPase H-ras depends on H-ras membrane conformer-specific nanoclustering.

Solution structures and biochemical data have provided a wealth of mechanistic insight into Ras GTPases. However, information on how much the membrane organization of these lipid-modified proteins impacts on their signaling is still scarce. Ras proteins are organized into membrane nanoclusters, which are necessary for Ras-MAPK signaling. Using quantitative conventional and super-resolution fluorescence methods, as well as mathematical modeling, we investigated nanoclustering of H-ras helix α4 and hypervariable region mutants that have different bona fide conformations on the membrane. By following the emergence of conformer-specific nanoclusters in the plasma membrane of mammalian cells, we found that conformers impart distinct nanoclustering responses depending on the cytoplasmic levels of the nanocluster scaffold galectin-1. Computational modeling revealed that complexes containing H-ras conformers and galectin-1 affect both the number and lifetime of nanoclusters and thus determine the specific Raf effector recruitment. Our results show that mutations in Ras can affect its nanoclustering response and thus allosterically effector recruitment and downstream signaling. We postulate that cancer- and developmental disease-linked mutations that are associated with the Ras membrane conformation may exhibit so far unrecognized Ras nanoclustering and therefore signaling alterations.

GTP-specific fab fragment-based GTPase activity assay.

GTPases are central cellular signaling proteins, which cycle between a GDP-bound inactive and a GTP-bound active conformation in a controlled manner. Ras GTPases are frequently mutated in cancer and so far only few experimental inhibitors exist. The most common methods for monitoring GTP hydrolysis rely on luminescent GDP- or GTP-analogs. In this study, the first GTP-specific Fab fragment and its application are described. We selected Fab fragments using the phage display technology. Six Fab fragments were found against 2'/3'-GTP-biotin and 8-GTP-biotin. Selected antibody fragments allowed specific detection of endogenous, free GTP. The most potent Fab fragment (2A4(GTP)) showed over 100-fold GTP-specificity over GDP, ATP, or CTP and was used to develop a heterogeneous time-resolved luminescence based assay for the monitoring of GTP concentration. The method allows studying the GEF dependent H-Ras activation (GTP binding) and GAP-catalyzed H-Ras deactivation (GTP hydrolysis) at nanomolar protein concentrations.

RhoGDI facilitates geranylgeranyltransferase-I-mediated RhoA prenylation.

Protein prenylation is a post-translational modification where farnesyl or geranylgeranyl groups are enzymatically attached to a C-terminal cysteine residue. This modification is essential for the activity of small cellular GTPases, as it allows them to associate with intracellular membranes. Dissociated from membranes, prenylated proteins need to be transported through the aqueous cytoplasm by protein carriers that shield the hydrophobic anchor from the solvent. One such carrier is Rho GDP dissociation inhibitor (RhoGDI). Recently, it was shown that prenylated Rho proteins that are not associated with RhoGDI are subjected to proteolysis in the cell. We hypothesized that the role of RhoGDI might be not only to associate with prenylated proteins but also to regulate the prenylation process in the cell. This idea is supported by the fact that RhoGDI binds both unprenylated and prenylated Rho proteins with high affinity in vitro, and hence, these interactions may affect the kinetics of prenylation. We addressed this question experimentally and found that RhoGDI increased the catalytic efficiency of geranylgeranyl transferase-I in RhoA prenylation. Nevertheless, we did not observe formation of a ternary RhoGDI∗RhoA∗GGTase-I complex, indicating sequential operation of geranylgeranyltransferase-I and RhoGDI. Our results suggest that RhoGDI accelerates Rho prenylation by kinetically trapping the reaction product, thereby increasing the rate of product release.

A homogeneous quenching resonance energy transfer assay for the kinetic analysis of the GTPase nucleotide exchange reaction.

A quenching resonance energy transfer (QRET) assay for small GTPase nucleotide exchange kinetic monitoring is demonstrated using nanomolar protein concentrations. Small GTPases are central signaling proteins in all eukaryotic cells acting as a "molecular switches" that are active in the GTP-state and inactive in the GDP-state. GTP-loading is highly regulated by guanine nucleotide exchange factors (GEFs). In several diseases, most prominently cancer, this process in misregulated. The kinetics of the nucleotide exchange reaction reports on the enzymatic activity of the GEF reaction system and is, therefore, of special interest. We determined the nucleotide exchange kinetics using europium-labeled GTP (Eu-GTP) in the QRET assay for small GTPases. After GEF catalyzed GTP-loading of a GTPase, a high time-resolved luminescence signal was found to be associated with GTPase bound Eu-GTP, whereas the non-bound Eu-GTP fraction was quenched by soluble quencher. The association kinetics of the Eu-GTP was measured after GEF addition, whereas the dissociation kinetics could be determined after addition of unlabeled GTP. The resulting association and dissociation rates were in agreement with previously published values for H-Ras(Wt), H-Ras(Q61G), and K-Ras(Wt), respectively. The broader applicability of the QRET assay for small GTPases was demonstrated by determining the kinetics of the Ect2 catalyzed RhoA(Wt) GTP-loading. The QRET assay allows the use of nanomolar protein concentrations, as more than 3-fold signal-to-background ratio was achieved with 50 nM GTPase and GEF proteins. Thus, small GTPase exchange kinetics can be efficiently determined in a HTS compatible 384-well plate format.

Protocol to measure and analyze protein interactions in mammalian cells using bioluminescence resonance energy transfer.

Bioluminescence resonance energy transfer (BRET) allows to quantitate protein interactions in intact cells. Here, we present a protocol for measuring BRET due to transient interactions of oncogenic K-RasG12V in plasma membrane nanoclusters of HEK293-EBNA cells. We describe steps for seeding, transfecting, and replating cells. We then detail procedures for their preparation for BRET measurements on a CLARIOstar microplate reader and detailed data analysis. For complete details on the use and execution of this protocol, please refer to Steffen et al.1.

RAS isoform specific activities are disrupted by disease associated mutations during cell differentiation.

The RAS-MAPK-pathway is aberrantly regulated in cancer and developmental diseases called RASopathies. While typically the impact of Ras on the proliferation of various cancer cell lines is assessed, it is poorly established how Ras affects cellular differentiation. Here we implement the C2C12 myoblast cell line to systematically study the effect of Ras mutants and Ras-pathway drugs on differentiation. We first provide evidence that a minor pool of Pax7+ progenitors replenishes a major pool of transit amplifying cells that are ready to differentiate. Our data indicate that Ras isoforms have distinct roles in the differentiating culture, where K-Ras depletion increases and H-Ras depletion decreases terminal differentiation. This assay could therefore provide significant new insights into Ras biology and Ras-driven diseases. In line with this, we found that all oncogenic Ras mutants block terminal differentiation of transit amplifying cells. By contrast, RASopathy associated K-Ras variants were less able to block differentiation. Profiling of eight targeted Ras-pathway drugs on seven oncogenic Ras mutants revealed their allele-specific activities and distinct abilities to restore normal differentiation as compared to triggering cell death. In particular, the MEK-inhibitor trametinib could broadly restore differentiation, while the mTOR-inhibitor rapamycin broadly suppressed differentiation. We expect that this quantitative assessment of the impact of Ras-pathway mutants and drugs on cellular differentiation has great potential to complement cancer cell proliferation data.

An Improved PDE6D Inhibitor Combines with Sildenafil To Inhibit KRAS Mutant Cancer Cell Growth.

The trafficking chaperone PDE6D (or PDEδ) was proposed as a surrogate target for K-Ras, leading to the development of a series of inhibitors that block its prenyl binding pocket. These inhibitors suffered from low solubility and suspected off-target effects, preventing their clinical development. Here, we developed a highly soluble, low nanomolar PDE6D inhibitor (PDE6Di), Deltaflexin3, which has the lowest off-target activity as compared to three prominent reference compounds. Deltaflexin3 reduces Ras signaling and selectively decreases the growth of KRAS mutant and PDE6D-dependent cancer cells. We further show that PKG2-mediated phosphorylation of Ser181 lowers K-Ras binding to PDE6D. Thus, Deltaflexin3 combines with the approved PKG2 activator Sildenafil to more potently inhibit PDE6D/K-Ras binding, cancer cell proliferation, and microtumor growth. As observed previously, inhibition of Ras trafficking, signaling, and cancer cell proliferation remained overall modest. Our results suggest reevaluating PDE6D as a K-Ras surrogate target in cancer.

Identification of an H-Ras nanocluster disrupting peptide.

Hyperactive Ras signalling is found in most cancers. Ras proteins are only active in membrane nanoclusters, which are therefore potential drug targets. We previously showed that the nanocluster scaffold galectin-1 (Gal1) enhances H-Ras nanoclustering via direct interaction with the Ras binding domain (RBD) of Raf. Here, we establish that the B-Raf preference of Gal1 emerges from the divergence of the Raf RBDs at their proposed Gal1-binding interface. We then identify the L5UR peptide, which disrupts this interaction by binding with low micromolar affinity to the B- and C-Raf-RBDs. Its 23-mer core fragment is sufficient to interfere with H-Ras nanoclustering, modulate Ras-signalling and moderately reduce cell viability. These latter two phenotypic effects may also emerge from the ability of L5UR to broadly engage with several RBD- and RA-domain containing Ras interactors. The L5UR-peptide core fragment is a starting point for the development of more specific reagents against Ras-nanoclustering and -interactors.

SHANK3 depletion leads to ERK signalling overdose and cell death in KRAS-mutant cancers.

The KRAS oncogene drives many common and highly fatal malignancies. These include pancreatic, lung, and colorectal cancer, where various activating KRAS mutations have made the development of KRAS inhibitors difficult. Here we identify the scaffold protein SH3 and multiple ankyrin repeat domain 3 (SHANK3) as a RAS interactor that binds active KRAS, including mutant forms, competes with RAF and limits oncogenic KRAS downstream signalling, maintaining mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) activity at an optimal level. SHANK3 depletion breaches this threshold, triggering MAPK/ERK signalling hyperactivation and MAPK/ERK-dependent cell death in KRAS-mutant cancers. Targeting this vulnerability through RNA interference or nanobody-mediated disruption of the SHANK3-KRAS interaction constrains tumour growth in vivo in female mice. Thus, inhibition of SHANK3-KRAS interaction represents an alternative strategy for selective killing of KRAS-mutant cancer cells through excessive signalling.

The effects of transmembrane sequence and dimerization on cleavage of the p75 neurotrophin receptor by γ-secretase.

Cleavage of transmembrane receptors by γ-secretase is the final step in the process of regulated intramembrane proteolysis (RIP) and has a significant impact on receptor function. Although relatively little is known about the molecular mechanism of γ-secretase enzymatic activity, it is becoming clear that substrate dimerization and/or the α-helical structure of the substrate can regulate the site and rate of γ-secretase activity. Here we show that the transmembrane domain of the pan-neurotrophin receptor p75(NTR), best known for regulating neuronal death, is sufficient for its homodimerization. Although the p75(NTR) ligands NGF and pro-NGF do not induce homerdimerization or RIP, homodimers of p75(NTR) are γ-secretase substrates. However, dimerization is not a requirement for p75(NTR) cleavage, suggesting that γ-secretase has the ability to recognize and cleave each receptor molecule independently. The transmembrane cysteine 257, which mediates covalent p75(NTR) interactions, is not crucial for homodimerization, but this residue is required for normal rates of γ-secretase cleavage. Similarly, mutation of the residues alanine 262 and glycine 266 of an AXXXG dimerization motif flanking the γ-secretase cleavage site within the p75(NTR) transmembrane domain alters the orientation of the domain and inhibits γ-secretase cleavage of p75(NTR). Nonetheless, heteromer interactions of p75(NTR) with TrkA increase full-length p75(NTR) homodimerization, which in turn potentiates the rate of γ-cleavage following TrkA activation independently of rates of α-cleavage. These results provide support for the idea that the helical structure of the p75(NTR) transmembrane domain, which may be affected by co-receptor interactions, is a key element in γ-secretase-catalyzed cleavage.

Quantitative analysis of prenylated RhoA interaction with its chaperone, RhoGDI.

Small GTPases of the Rho family regulate cytoskeleton remodeling, cell polarity, and transcription, as well as the cell cycle, in eukaryotic cells. Membrane delivery and recycling of the Rho GTPases is mediated by Rho GDP dissociation inhibitor (RhoGDI), which forms a stable complex with prenylated Rho GTPases. We analyzed the interaction of RhoGDI with the active and inactive forms of prenylated and unprenylated RhoA. We demonstrate that RhoGDI binds the prenylated form of RhoA·GDP with unexpectedly high affinity (K(d) = 5 pm). The very long half-life of the complex is reduced 25-fold on RhoA activation, with a concomitant reduction in affinity (K(d) = 3 nm). The 2.8-Å structure of the RhoA·guanosine 5'-[ß,γ-imido] triphosphate (GMPPNP)·RhoGDI complex demonstrated that complex formation forces the activated RhoA into a GDP-bound conformation in the absence of nucleotide hydrolysis. We demonstrate that membrane extraction of Rho GTPase by RhoGDI is a thermodynamically favored passive process that operates through a series of progressively tighter intermediates, much like the one that is mediated by RabGDI.

Ski-interacting protein (SKIP) interacts with androgen receptor in the nucleus and modulates androgen-dependent transcription.

BACKGROUND: The androgen receptor (AR) is a member of the nuclear receptor (NR) superfamily of ligand-inducible DNA transcription factors, and is the major mediator of male sexual development, prostate growth and the pathogenesis of prostate cancer. Cell and gene specific regulation by the AR is determined by availability of and interaction with sets of key accessory cofactors. Ski-interacting protein (SKIP; SNW1, NCOA62) is a cofactor shown to interact with several NRs and a diverse range of other transcription factors. Interestingly, SKIP as part of the spliceosome is thought to link mRNA splicing with transcription. SKIP has not been previously shown to interact with the AR. RESULTS: The aim of this study was to investigate whether SKIP interacts with the AR and modulates AR-dependent transcription. Here, we show by co-immunoprecipitation experiments that SKIP is in a complex with the AR. Moreover, SKIP increased 5α-dihydrotestosterone (DHT) induced N-terminal/C-terminal AR interaction from 12-fold to almost 300-fold in a two-hybrid assay, and enhanced AR ligand-independent AF-1 transactivation. SKIP augmented ligand- and AR-dependent transactivation in PC3 prostate cancer cells. Live-cell imaging revealed a fast (half-time=129 s) translocation of AR from the cytoplasm to the nucleus upon DHT-stimulation. Förster resonance energy transfer (FRET) experiments suggest a direct AR-SKIP interaction in the nucleus upon translocation. CONCLUSIONS: Our results suggest that SKIP interacts with AR in the nucleus and enhances AR-dependent transactivation and N/C-interaction supporting a role for SKIP as an AR co-factor.