Mig-6 is a negative regulator of the epidermal growth factor receptor signal

In contrast to signal generation and transmission, the mechanisms and molecules that negatively regulate receptor tyrosine kinase (RTK) signaling are poorly understood. Here we characterize Mig-6 as a novel negative feedback regulator of the epidermal growth factor receptor (EGFR) and potential tumor suppressor. Mig-6 was identified in a yeast two-hybrid screen with the kinase active domain of the EGFR as bait. Upon EGF stimulation Mig-6 binds to the EGFR involving a highly acidic region between amino acids 985-995. This interaction is kinase activity-dependent, but independent of tyrosine 992. Mig-6 overexpression results in reduced activation of the mitogenactivated protein kinase ERK2 in response to EGF, but not FGF or PDGF, stimulation and in enhanced receptor internalization without affecting the rate of degradation. The induction of Mig-6 mRNA expression in response to EGF, but not FGF, indicates the existence of a negative regulatory feedback loop. Consistent with these findings, a possible role as tumor suppressor is indicated by Mig-6-mediated inhibition of EGFR overexpression-induced transformation of Rati cells.     

Mig-6 participates in the regulation of cell senescence and retinoblastoma protein phosphorylation

Mitogen-inducible gene-6 (Mig-6) is a cytosolic multiadaptor protein that is best known for its role as a negative feedback regulator of epidermal growth factor receptor (EGFR) mediated signalling. Alternative roles of Mig-6 are becoming increasingly recognised. Consistently with this, Mig-6 was demonstrated to be involved in a broad spectrum of cellular events including tumour suppression which may include the induction of cellular senescence. Here, we investigated the mechanisms of Mig-6 induced premature cell senescence. Endogenous Mig-6 is poorly expressed in young fibroblasts, whilst its expression rises in cells presenting with typical features of senescence. Overexpression of Mig-6 is sufficient to trigger premature cellular senescence of early passage diploid lung fibroblasts (WI-38). Interestingly, Mig-6 overexpression reduced retinoblastoma protein (pRb) phosphorylation at the inactivating Ser249/Thr252 sites. We also found that phosphorylation of these sites in pRb is increased in the presence of the B-Raf V600E oncogenic mutation. We further show that Mig-6 overexpression reduces B-Raf V600E mediated pRb inactivation and preserves pRb function.     

Transmodulation of epidermal growth factor receptor function by cyclic AMP-dependent protein kinase.

Binding of epidermal growth factor (EGF) to its receptor (EGFR) augments the tyrosine kinase activity of the receptor and autophosphorylation. Exposure of some tissues and cells to EGF also stimulates adenylyl cyclase activity and results in an increase in cyclic AMP (cAMP) levels. Because cAMP activates the cAMP-dependent protein kinase A (PKA), we investigated the effect of PKA on the EGFR. The purified catalytic subunit of PKA (PKAc) stoichiometrically phosphorylated the purified full-length wild type (WT) and kinase negative (K721M) forms of the EGFR. PKAc phosphorylated both WT-EGFR as well as a mutant truncated form of EGFR (Delta1022-1186) exclusively on serine residues. Moreover, PKAc also phosphorylated the cytosolic domain of the EGFR (EGFRKD). Phosphorylation of the purified WT as well as EGFRDelta1022-1186 and EGFRKD was accompanied by decreased autophosphorylation and diminished tyrosine kinase activity. Pretreatment of REF-52 cells with the nonhydrolyzable cAMP analog, 8-(4-chlorophenylthio)-cAMP, decreased EGF-induced tyrosine phosphorylation of cellular proteins as well as activation of the WT-EGFR. Similar effects were also observed in B82L cells transfected to express the Delta1022-1186 form of EGFR. Furthermore, activation of PKAc in intact cells resulted in serine phosphorylation of the EGFR. The decreased phosphorylation of cellular proteins and diminished activation of the EGFR in cells treated with the cAMP analog was not the result of altered binding of EGF to its receptors or changes in receptor internalization. Therefore, we conclude that PKA phosphorylates the EGFR on Ser residues and decreases its tyrosine kinase activity and signal transduction both in vitro and in vivo.     

MAPK scaffold IQGAP1 binds the EGF receptor and modulates its activation

Cellular responses produced by EGF are mediated through the receptor (EGFR) and by various enzymes and scaffolds. Recent studies document IQGAP1 as a scaffold for the MAPK cascade, binding directly to B-Raf, MEK, and ERK and regulating their activation in response to EGF. We previously showed that EGF is unable to activate B-Raf in cells lacking IQGAP1. However, the mechanism by which IQGAP1 links B-Raf to EGFR was unknown. Here we report that endogenous EGFR and IQGAP1 co-localize and co-immunoprecipitate in cells. EGF has no effect on the association, but Ca(2+) attenuates binding. In vitro analysis demonstrated a direct association mediated through the IQ and kinase domains of IQGAP1 and EGFR, respectively. Calmodulin disrupts this interaction. Using a mass spectrometry-based assay, we show that EGF induces phosphorylation of IQGAP1 Ser(1443), a residue known to be phosphorylated by PKC. This phosphorylation is eliminated by pharmacological inhibition of either EGFR or PKC and transfection with small interfering RNA directed against the PKCα isoform. In IQGAP1-null cells, EGF-stimulated tyrosine phosphorylation of EGFR is severely attenuated. Normal levels of autophosphorylation are restored by reconstituting wild type IQGAP1 and enhanced by an IQGAP1 S1443D mutant. Collectively, these data demonstrate a functional interaction between IQGAP1 and EGFR and suggest that IQGAP1 modulates EGFR activation.     

EGF receptor crosstalks with cytokine receptors leading to the activation of c-Jun kinase in response to UV irradiation in human keratinocytes

Ultraviolet (UV) irradiation causes photoageing through induction of matrix-degrading metalloproteinases (MMP), which are upregulated by activator protein-1 (AP-1) (Jun/Fos). The c-Jun kinase activity proves to be critically important in the regulation of AP-1 activity. Our previous studies showed that UV irradiation activates epidermal growth factor receptor (EGFR) and cytokine receptors leading to the activation of c-Jun kinase in cultured human skin keratinocytes in vitro and in human skin in vivo. However, the mechanism of UV-induced cell surface receptor activation and the crosstalk among growth factor receptor and cytokine receptors were not fully investigated. This study showed that UV (30 mJ/cm(2))-induced EGFR tyrosine phosphorylation in a manner similar to EGF (100 ng/ml), or IL-1beta (10 ng/ml) in cultured human keratinocytes. In all cases, EGFR tyrosine phosphorylation was completely inhibited by pretreatment of PD153035 (100 nM, 1 h). Also observed was that UV induced autophosphorylation of interleukin 1 receptor associated kinase (IRAK) in a manner analogous to IL-1beta or EGF. In both UV and EGF cases, the phosphorylation of IRAK was inhibited by pretreatment of PD153035. However, IL-1beta-induced IRAK activation was not affected by PD153035. In vitro kinase assay using GST-c-Jun as a substrate revealed that pretreatment of PD153035 completely inhibited UV- and IL-1-induced c-Jun kinase activity in cultured keratinocytes. Taken together, the above data suggest that EGFR plays dominant role in the crosstalk among growth factor receptor and cytokine receptors leading to the activation of c-Jun kinase upon UV irradiation, and that EGFR could be one of the targets for clinical and cosmetical prevention of UV-induced skin aging.     

Chronic ethanol consumption disrupts complexation between EGF receptor and phospholipase C-gamma1: relevance to impaired hepatocyte proliferation

We have previously shown that chronic ethanol consumption inhibits liver regeneration by impairing EGF receptor (EGFR)-operated phospholipase C-gamma1 (PLC-gamma1) activation and resultant intracellular Ca2+ signalling. Activation of PLC-gamma1 by EGFR requires the EGFR to bind to PLC-gamma1 after its translocation from cytosol to cytoskeleton. In order to understand the mechanism by which ethanol impairs PLC-gamma1 activation, we examined the effect of alcohol on interactions between EGFR and PLC-gamma1. In cultured hepatocytes from control rats, EGF rapidly induced tyrosine phosphorylation of both the EGFR and of PLC-gamma1. EGF also stimulated PLC-gamma1 translocation from cytosol to a cytoskeletal compartment where PLC-gamma1 interacted with EGFR. In hepatocytes from rats fed ethanol for 16 weeks, the above reactions were substantially inhibited. Tyrphostin AG1478, an EGFR-specific tyrosine kinase inhibitor, mimicked the effects of chronic ethanol on EGFR phosphorylation, PLC-gamma1 translocation and interactions between EGFR and PLC-gamma1 in the cytoskeleton. Further, tyrphostin AG1478 also inhibited EGF-induced DNA synthesis. These results indicate that ethanol impairs EGFR-operated [Ca2+]i signaling by disrupting the interactions between EGFR and PLC-gamma1.     

Activated Cdc42-associated kinase 1 is a component of EGF receptor signaling complex and regulates EGF receptor degradation

Cdc42-associated tyrosine kinase 1 (ACK1) is a specific down-stream effector of Cdc42, a Rho family small G-protein. Previous studies have shown that ACK1 interacts with clathrin heavy chain and is involved in clathrin-coated vesicle endocytosis. Here we report that ACK1 interacted with epidermal growth factor receptor (EGFR) upon EGF stimulation via a region at carboxy terminus that is highly homologous to Gene-33/Mig-6/RALT. The interaction of ACK1 with EGFR was dependent on the kinase activity or tyrosine phosphorylation of EGFR. Immunofluorescent staining using anti-EGFR and GFP-ACK1 indicates that ACK1 was colocalized with EGFR on EEA-1 positive vesicles upon EGF stimulation. Suppression of the expression of ACK1 by ACK-RNAi inhibited ligand-induced degradation of EGFR upon EGF stimulation, suggesting that ACK1 plays an important role in regulation of EGFR degradation in cells. Furthermore, we identified ACK1 as an ubiquitin-binding protein. Through an ubiquitin-association (Uba) domain at the carboxy terminus, ACK1 binds to both poly- and mono-ubiquitin. Overexpression of the Uba domain-deletion mutant of ACK1 blocked the ligand-dependent degradation of EGFR, suggesting that ACK1 regulates EGFR degradation via its Uba domain. Taken together, our studies suggest that ACK1 senses signal of EGF and regulates ligand-induced degradation of EGFR.     

Regulation of the antioncogenic Chk2 kinase by the oncogenic Wip1 phosphatase

The antioncogenic Chk2 kinase plays a crucial role in DNA damage-induced cell-cycle checkpoint regulation. Here we show that Chk2 associates with the oncogenic protein Wip1 (wild-type p53-inducible phosphatase 1) (PPM1D), a p53-inducible protein phosphatase. Phosphorylation of Chk2 at threonine68 (Thr68), a critical event for Chk2 activation, which is normally induced by DNA damage or overexpression of Chk2, is inhibited by expression of wild-type (WT), but not a phosphatase-deficient mutant (D314A) of Wip1 in cultured cells. Furthermore, an in vitro phosphatase assay revealed that Wip1 (WT), but not Wip1 (D314A), dephosphorylates Thr68 on phosphorylated Chk2 in vitro, resulting in the inhibition of Chk2 kinase activity toward glutathione S-transferase-Cdc25C. Moreover, inhibition of Wip1 expression by RNA interference results in abnormally sustained Thr68 phosphorylation of Chk2 and increased susceptibility of cells in response to DNA damage, indicating that Wip1 acts as a negative regulator of Chk2 in response to DNA damage.     

Phosphorylation of RhoGDI by Pak1 mediates dissociation of Rac GTPase

Selective activation of Rac GTPase signaling pathways requires the specific release of Rac from RhoGDI complexes. We identified a RhoGDI kinase from bovine brain as p21-activated kinase (Pak). Pak1 binds and phosphorylates RhoGDI both in vitro and in vivo at Ser101 and Ser174. This resulted in dissociation of Rac1-RhoGDI, but not RhoA-RhoGDI, complexes, as determined by in vitro assays of complexation and in vivo by coimmunoprecipitation analysis. We observed that Cdc42-induced Rac1 activation is inhibited by expression of Pak1 autoinhibitory domain. The dissociation of Rac1 from RhoGDI and its subsequent activation stimulated by PDGF or EGF is also attenuated by Pak1 autoinhibitory domain, and this is dependent on the ability of RhoGDI to be phosphorylated at Ser101/174. These results support a role for Pak1-mediated RhoGDI phosphorylation as a mechanism for Cdc42-mediated Rac activation, and suggest the possibility of Rac-induced positive feed-forward regulation of Rac activity.     

Helicobacter pylori activate epidermal growth factor receptor- and phosphatidylinositol 3-OH kinase-dependent Akt and glycogen synthase kinase 3beta phosphorylation

The signalling pathways leading to the development of Helicobacter pylori -induced gastric cancer remain poorly understood. We tested the hypothesis that H. pylori infections involve the activation of Akt signalling in human gastric epithelial cancer cells. Immunoblot, immunofluorescence and kinase assays show that H. pylori infection of gastric epithelial cells induced phosphorylation of Akt at Ser 473 and Thr 308. Mutations in the H. pylori virulence factor OipA dramatically reduced phosphorylation of Ser 473, while the cag pathogenicity island mutants predominantly inhibited phosphorylation of Thr 308. As the downstream of Akt activation, H. pylori infection inactivated the inactivation of glycogen synthase kinase 3β at Ser 9 by its phosphorylation. As the upstream of Akt activation, H. pylori infection activated epidermal growth factor receptor (EGFR) at Tyr 992, phosphatidylinositol 3-OH kinase (PI3K) p85 subunit and PI3K-dependent kinase 1 at Ser 241. Pharmacologic inhibitors of PI3K or mitogen-activated protein kinase kinase (MEK), Akt knock-down and EGFR knock-down showed that H. pylori infection induced the activation of EGFR→PI3K→PI3K-dependent kinase 1→Akt→extracellular signal-regulated kinase signalling pathways, the inactivation of glycogen synthase kinase 3β and interleukin-8 production. The combined functions of cag pathogenicity island and OipA were necessary and sufficient for full activation of signalling at each level. We propose activation of these pathways as a novel mechanism for H. pylori -mediated carcinogenesis.     

Inhibition of Chk1 by activated PKB/Akt

We have shown recently that DNA damage effector kinase Chk1 is phosphorylated in vitro by protein kinase B/Akt (PKB/Akt) on serine 280. Activation of Chk1 by DNA damage in vivo is suppressed in presence of activated PKB. In this study we show that Chk1 is phosphorylated by PKB in vivo, and that increased phosphorylation by PKB on serine 280 correlates with impairment of Chk1 activation by DNA damage. Our results indicate a likely mechanism for the negative effects that phosphorylation of serine 280 has on activation of Chk1. The Chk1 protein phosphorylated by PKB on serine 280 does not enter into protein complexes after replication arrest. Moreover, Chk1 phosphorylated by PKB fails to undergo activating phosphorylation on serine 345 by ATM/ATR. Phosphorylation by ATM/ATR and association with other checkpoint proteins are essential steps in activation of Chk1. Inhibition of these steps provides a plausible explanation for the observed attenuation of Chk1 activation by activated PKB after DNA damage.     

Replication stress activates DNA polymerase alpha-associated Chk1

Chk1 contributes to both intra-S and DNA damage checkpoint responses. Here, we show that depletion of DNA Polα and not Polε or Polδ by siRNA induces phosphorylation of Chk1 on Ser345, thus phenocopying antimetabolite exposure. Combinatorial ablation of DNA Polα and Chk1 causes an accumulation of γ-H2A.X, a marker of double-strand DNA breaks, suggesting that activation of Chk1 in this context is essential for suppression of DNA damage. Co-depletion of DNA Polα with ATR yields similar phenotypes, suggesting that ATR and Chk1 are epistatic and required for maintenance of genomic integrity following replication stress. Significantly, Chk1 and DNA Polα can be co-immunoprecipated from native cell extracts. Moreover, following replication stress, Polα-associated Chk1 becomes rapidly phosphorylated on Ser345 in a TopBP1 and ATR-dependent manner. Hence, the ability to efficiently phosphorylate Chk1 in the context of DNA Polα complexes is correlated with suppression of DNA damage following replication stress. These findings identify DNA Polα as an important component of the signal transduction cascade that activates the intra-S checkpoint.     

Structural and functional versatility of the FHA domain in DNA-damage signaling by the tumor suppressor kinase Chk2

The Chk2 Ser/Thr kinase plays crucial, evolutionarily conserved roles in cellular responses to DNA damage. Identification of two pro-oncogenic mutations within the Chk2 FHA domain has highlighted its importance for Chk2 function in checkpoint activation. The X-ray structure of the Chk2 FHA domain in complex with an in vitro selected phosphopeptide motif reveals the determinants of binding specificity and shows that both mutations are remote from the peptide binding site. We show that the Chk2 FHA domain mediates ATM-dependent Chk2 phosphorylation and targeting of Chk2 to in vivo binding partners such as BRCA1 through either or both of two structurally distinct mechanisms. Although phospho-dependent binding is important for Chk2 activity, previously uncharacterized phospho-independent FHA domain interactions appear to be the primary target of oncogenic lesions.     

The activation of eukaryotic initiation factor (eIF)2B by growth factors in PC12 cells requires MEK/ERK signalling

Epidermal and nerve growth factors (EGF and NGF) activate protein synthesis and initiation factor eIF2B in rat phaeochromocytoma (PC12) cells. The activation of protein synthesis by EGF or NGF depends upon extracellular regulated kinase kinase (MEK)/extracellular regulated kinase signalling. Here we show that PD98059, an inhibitor of MEK activation, blocks the activation of eIF2B by EGF or NGF. It is known that eIF2B activity can be inhibited by phosphorylation at Ser535 in its epsilon-subunit by glycogen synthase kinase (GSK)-3. We find that inactivation of GSK-3 by EGF or NGF is blocked by PD98059. However, neither EGF nor NGF caused a detectable change in phosphorylation of Ser535 of eIF2Bepsilon. Thus, the EGF- and NGF-induced activation of eIF2B in PC12 cells involves regulatory mechanisms distinct from dephosphorylation of the GSK-3 site.     

ATR-mediated checkpoint pathways regulate phosphorylation and activation of human Chk1

Chk1 is an evolutionarily conserved protein kinase that regulates cell cycle progression in response to checkpoint activation. In this study, we demonstrated that agents that block DNA replication or cause certain forms of DNA damage induce the phosphorylation of human Chk1. The phosphorylated form of Chk1 possessed higher intrinsic protein kinase activity and eluted more quickly on gel filtration columns. Serines 317 and 345 were identified as sites of phosphorylation in vivo, and ATR (the ATM- and Rad3-related protein kinase) phosphorylated both of these sites in vitro. Furthermore, phosphorylation of Chk1 on serines 317 and 345 in vivo was ATR dependent. Mutants of Chk1 containing alanine in place of serines 317 and 345 were poorly activated in response to replication blocks or genotoxic stress in vivo, were poorly phosphorylated by ATR in vitro, and were not found in faster-eluting fractions by gel filtration. These findings demonstrate that the activation of Chk1 in response to replication blocks and certain forms of genotoxic stress involves phosphorylation of serines 317 and 345. In addition, this study implicates ATR as a direct upstream activator of Chk1 in human cells.     

EGF signalling amplification induced by dynamic clustering of EGFR

Lateral interaction is an important feature of various types of cell surface receptors including the receptor tyrosine kinases (RTKs). Here we report that dynamic lateral interaction produces amplification and variation in signalling of the EGF receptor, a member of RTKs. Binding of EGF is known to induce transphosphorylation inside EGFR dimers. Using single-molecule techniques, the relationship between EGF binding and EGFR phosphorylation has been determined. The number of phosphorylated EGFR molecules became larger than that of EGF binding as unliganded EGFR was phosphorylated, meaning an amplification of EGF signalling. EGFR formed clusters continuously exchanging their elements through thermal diffusion, and direct and/or indirect lateral interactions. As a result, various types of activation sites differing in number of activated receptors were generated. Amplification required no cytoplasmic factors and was observed on semi-intact cells for a wide range of number of EGFR molecules (10(4)-10(6) per cell) suggesting generality of this process.     

The androgen receptor associates with the epidermal growth factor receptor in androgen-sensitive prostate cancer cells

Many recent evidences indicate that androgen-sensitive prostate cancer cells have a lower malignant phenotype that is in particular characterized by a reduced migration and invasion. We previously demonstrated that expression of androgen receptor (AR) by transfection of the androgen-independent prostate cancer cell line PC3 decreases invasion and adhesion of these cells (PC3-AR) through modulation of alpha6beta4 integrin expression. The treatment with the synthetic androgen R1881 further reduced invasion of the cells without, however, modifying alpha6beta4 expression on the cell surface, suggesting an interference with the invasion process in response to EGF. We investigated whether the presence of the AR could affect EGF receptor (EGFR)-mediated signaling in response to EGF by evaluating autotransphosphorylation of the receptor as well as activation of downstream signalling pathways. Immunoprecipitation studies demonstrated a reduction of EGF-induced tyrosine phosphorylation of EGFR in PC3-AR cells. In addition, EGF-stimulated PI3K activity, a key signalling pathway for invasion of these cells, was decreased in PC3-AR cells and further reduced by treatment with R1881, indicating decreased functionality of EGFR. An interaction between EGFR and AR has been demonstrated by immunoconfocal and co-immunoprecipitation analysis in PC3-AR cells, suggesting a possible interference of AR on EGFR signalling by interaction of the two proteins. In conclusion, our results suggest that the expression of AR by transfection in PC3 cells confers a less malignant phenotype by interfering with EGFR autophosphorylation and signalling in response to EGF leading to invasion through a mechanism involving an interaction between AR and EGFR.     

EGF transregulates opioid receptors through EGFR-mediated GRK2 phosphorylation and activation

G protein-coupled receptor (GPCR) kinases (GRKs) are key regulators of GPCR function. Here we demonstrate that activation of epidermal growth factor receptor (EGFR), a member of receptor tyrosine kinase family, stimulates GRK2 activity and transregulates the function of G protein-coupled opioid receptors. Our data showed that EGF treatment promoted DOR internalization induced by DOR agonist and this required the intactness of GRK2-phosphorylation sites in DOR. EGF stimulation induced the association of GRK2 with the activated EGFR and the translocation of GRK2 to the plasma membrane. After EGF treatment, GRK2 was phosphorylated at tyrosyl residues. Mutational analysis indicated that EGFR-mediated phosphorylation occurred at GRK2 N-terminal tyrosyl residues previously shown as c-Src phosphorylation sites. However, c-Src activity was not required for EGFR-mediated phosphorylation of GRK2. In vitro assays indicated that GRK2 was a direct interactor and a substrate of EGFR. EGF treatment remarkably elevated DOR phosphorylation in cells expressing the wild-type GRK2 in an EGFR tyrosine kinase activity-dependent manner, whereas EGF-stimulated DOR phosphorylation was greatly decreased in cells expressing mutant GRK2 lacking EGFR tyrosine kinase sites. We further showed that EGF also stimulated internalization of mu-opioid receptor, and this effect was inhibited by GRK2 siRNA. These data indicate that EGF transregulates opioid receptors through EGFR-mediated tyrosyl phosphorylation and activation of GRK2 and propose GRK2 as a mediator of cross-talk from RTK to GPCR signaling pathway.     

Cholesterol depletion from the plasma membrane triggers ligand-independent activation of the epidermal growth factor receptor

We recently demonstrated that depletion of plasma membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) caused activation of MAPK (Chen, X., and Resh, M. D. (2001) J. Biol. Chem. 276, 34617-34623). MAPK activation was phosphatidylinositol 3-kinase (PI3K)-dependent and involved increased tyrosine phosphorylation of the p85 subunit of PI3K. We next determined whether MbetaCD treatment induced tyrosine phosphorylation of other cellular proteins. Here we report that cholesterol depletion of serum-starved COS-1 cells with MbetaCD or filipin caused an increase in Tyr(P) levels of a 180-kDa protein that was identified as the epidermal growth factor receptor (EGFR). Cross-linking experiments showed that MbetaCD induced dimerization of EGFR, indicative of receptor activation. Reagents that block release of membrane-bound EGFR ligands did not affect MbetaCD-induced tyrosine phosphorylation of EGFR, indicating that MbetaCD activation of EGFR is ligand-independent. Moreover, MbetaCD treatment resulted in increased tyrosine phosphorylation of EGFR downstream targets and Ras activation. Incubation of cells with the specific EGFR inhibitor AG4178 blocked MbetaCD-induced phosphorylation of EGFR, SHC, phospholipase C-gamma, and Gab-1 as well as MAPK activation. We conclude that cholesterol depletion from the plasma membrane by MbetaCD causes ligand-independent activation of EGFR, resulting in MAPK activation by PI3K and Ras-dependent mechanisms. Moreover, these studies reveal a novel mode of action of MbetaCD, in addition to its ability to disrupt membrane rafts.