In vivo fluorescence microscopy studies of bacterial cells have shown that the bacterial shape-determining protein and actin homolog, MreB, forms cable-like structures that spiral around the periphery of the cell. The molecular structure of these cables has yet to be established. Here we show by electron microscopy that Thermatoga maritime MreB forms complex, several micrometer long multilayered sheets consisting of diagonally interwoven filaments in the presence of either ATP or GTP. This architecture, in agreement with recent rheological measurements on MreB cables, may have superior mechanical properties and could be an important feature for maintaining bacterial cell shape. MreB polymers within the sheets appear to be single-stranded helical filaments rather than the linear protofilaments found in the MreB crystal structure. Sheet assembly occurs over a wide range of pH, ionic strength and temperature. Polymerization kinetics are consistent with a cooperative assembly mechanism requiring only two steps: monomer activation followed by elongation. Steady state TIRF microscopy studies of MreB suggest filament treadmilling whilst high pressure small angle X-ray scattering measurements indicate that the stability of MreB polymers is similar to that of F-actin filaments. In the presence of ADP or GDP long, thin cables formed, in which MreB was arranged in parallel as linear protofilaments. This suggests that the bacterial cell may exploit various nucleotides to generate different filament structures within cables for specific MreB-based functions.

When cells attach to the extracellular matrix (ECM) a proliferation permissive signal is engaged. The mechanism involves activation of the integrin/PI3K/Akt signal pathway. FoxO3a is a transcriptional activator and inhibits cell proliferation via up-regulating the expression of the cell cycle inhibitor p27. Furthermore, it is known that activated Akt can suppress FoxO3a function. However, it is not known whether integrin interaction with the ECM regulates FoxO3a function. We examined whether the beta1-integrin-mediated signaling pathway promotes fibroblast proliferation via FoxO3a suppression. We found that when fibroblasts are attached to the ECM, PTEN protein expression and activity are inhibited due to promotion of PTEN degradation. This decrease in PTEN function permits FoxO3a suppression via PI3K/Akt pathway. In contrast, the inhibition of PI3K/Akt or reconstitution of PTEN restores FoxO3a expression on collagen. Furthermore, we found that the serine/threonine phosphatase PP2A also regulates FoxO3a. PP2A expression/activity is low when fibroblasts are attached to collagen, and PP2A over-expression augments FoxO3a levels. Thus the mechanism involves a coordinated decrease in PTEN and PP2A phosphatase activity and increase in PI3K/Akt activity. We show that beta1 integrin-ECM interaction decreases FoxO3a protein levels via caspase-3 mediated cleavage. Our novel finding indicates that during fibroblast interaction with ECM, activation of beta1-integrin/PI3K/Akt by inhibiting PTEN in combination with low PP2A phosphatase activity synergistically inhibits FoxO3a, promoting fibroblast proliferation.

ARF6 regulates the endocytosis and recycling of a variety of proteins, and also promotes peripheral actin rearrangements and cell motility. ARF6 is activated by a large number of GEFs, which likely regulate ARF6 at different locations and during different processes. In this study, we investigate the roles of the cytohesin ARF-GEFs during the recycling of beta integrin 1. Intriguingly, we find that knockdown and overexpression of ARNO/cytohesin 2 and GRP1/cytohesin 3 have opposing effects on cell adhesion and spreading on fibronectin and on cell migration. We find that ARNO/cytohesin 2 is required for integrin beta1 recycling while GRP1/cytohesin 3 is dispensable for this process. This is the first demonstration of unique roles for these proteins.

Reactive oxygen species generated by activated neutrophils can cause oxidative stress and tissue damage. S100A8 (A8) and S100A9 (A9), abundant in neutrophil cytoplasm, are exquisitely sensitive to oxidation, which may alter their functions. Murine A8 is a neutrophil chemoattractant but it suppresses leukocyte transmigration in the microcirculation when S-nitrosylated. Glutathione (GSH) modulates intracellular redox, and S-glutathionylation can protect susceptible proteins from oxidative damage and regulate function. We characterized S-glutathionylation of A9; GSSG and GSNO generated S-glutathionylated A8 (A8-SSG) and A9 (A9-SSG) in vitro, whereas only A9-SSG was detected in cytosol of neutrophils activated with PMA, but not with fMLP or opsonized zymosan. S-glutathionylation exposed more hydrophobic regions in Zn2+-bound A9 but did not alter Zn2+-binding affinity. A9-SSG had reduced capacity to form heterocomplexes with A8 but the arachidonic acid binding capacities of A8/A9 and A8/A9-SSG were similar. A9 and A8/A9 bind endothelial cells; S-glutathionylation reduced binding. We found little effect of A9 or A9-SSG on neutrophil CD11b/CD18 expression, or neutrophil adhesion to endothelial cells. However, A9, A9-SSG and A8/A9 promoted neutrophil adhesion to fibronectin but, in the presence of A8, A9-mediated adhesion was abrogated by glutathionylation. S-glutathionylation of A9 may protect its oxidation to higher oligomers, and reduce neutrophil binding to the extracellular matrix. This may regulate the magnitude of neutrophil migration in the extravasculature, and together with the functional changes we reported for S-nitrosylated A8, particular oxidative modifications of these proteins may limit tissue damage in acute inflammation.

Lipoxygenases are enzymes that are found ubiquitously in higher animals and plants, but have only recently been identified in a number of bacteria. The genome of the diazotrophic unicellular cyanobacterium Cyanothece sp. harbors two genes with homology to lipoxygenases. Here we describe the isolation of one gene, formerly named csplox2. It was cloned and the protein was expressed in E. coli and purified. The purified enzyme belongs to the group of prokaryotic mini lipoxygenases, because it had a molecular weight of 65 kDa. Interestingly, it catalyzed the conversion of linoleic acid, the only endogenously found polyunsaturated fatty acid, primarily to the bisallylic hydroperoxide 11-hydroperoxy octadecadienoic acid. This product had previously only been described for the manganese lipoxygenase from the take all fungus, Gaeumannomyces graminis. By contrast, CspLOX2 was shown to be an iron lipoxygenase. In addition, CspLOX2 formed a mixture of typical conjugated lipoxygenase products, e.g. 9R- and 13S-hydroperoxide. The conversion of linoleic acid took place with a maximum reaction rate of 31 s-1. Incubation of the enzyme with [(11S)-2H]-linoleic acid led to the formation of hydroperoxides which had lost the deuterium label, thus suggesting that CspLOX2 catalyzes antarafacial oxygenation as opposed to the mechanism of manganese lipoxygenase. CspLOX2 could also oxidize diarachidonylglycerophosphatidylcholine with similar specificity as the free fatty acid, indicating that binding of the substrate takes place with a "tail-first" orientation. We conclude that CspLOX2 is a novel iron mini-lipoxygenase that catalyzes the formation of bisallylic hydroperoxide as major product.

Eukaryotic cells dynamically reorganize the actin cytoskeleton to regulate various cellular activities, such as cell shape change, cell motility, cytokinesis, and vesicular transport. Diaphanous-related formins (DRFs), such as Daam1 and mDia1, play central roles in actin dynamics through assembling linear actin filaments. It has been reported that the GTP-bound active Rho binds directly to DRFs and partially unleashes the intramolecular autoinhibition of DRFs. However, whether proteins other than Rho involve the regulation of the actin assembly activity of DRFs has been unclear. Here, we show that Flightless-I (Fli-I), a gelsolin family protein essential for early development, binds directly to Daam1 and mDia1. Fli-I enhances the intrinsic actin assembly activity of Daam1 and mDia1 in vitro and is required for Daam1-induced actin assembly in living cells. Furthermore, Fli-I promotes the GTP-bound active Rho-mediated relief of the autoinhibition of Daam1 and mDia1. Thus, Fli-I is a novel positive regulator of Rho-induced linear actin assembly mediated by DRFs.

How most apoptotic stimuli trigger mitochondrial dysfunction remains to be resolved. We screened the entire Bcl-2 network for its involvement in DNA damage-induced apoptosis in HeLa cells. While the anti-apoptotic member Bcl-xL served as a major suppressor, apoptosis initiated only when both Mcl-1 and Bcl-xL were eliminated. The pro-apoptotic members Bak, Bad, Bim, and Noxa were required for apoptosis induced by DNA damaging agents camptothecin and UV. We used a tagged Bcl-xL expression system to capture the relevant BH3-only proteins that bind to Bcl-xL in response to DNA damage. Surprisingly, unlike Bad and Bim, which bound Bcl-xL constitutively, Noxa became Mcl-1 free, and interacted with Bcl-xL after DNA damage, but not after death receptor engagement. Similar observations were also made in A431 cells. Importantly, this induced interaction caused cytochrome c release and apoptosis, and was directly inhibited by Mcl-1, a protein eliminated or inactivated following DNA damage. These results suggest that the loss/inactivation of Mcl-1 in conjunction with an induced Noxa/Bcl-xL interaction may serve as a trigger for mitochondrial dysfunction during DNA damage-induced apoptosis.

Inactivation of the retinoblastoma protein through phosphorylation is an important step in promoting cell cycle progression, and hyperphosphorylated Rb is commonly found in tumors. Rb phosphorylation prevents its association with the E2F transcription factor; however, the molecular basis for complex inhibition has not been established. We identify here the key phosphorylation events and conformational changes that occur in Rb to inhibit the specific association between the E2F transactivation domain (E2FTD) and the Rb pocket domain. Calorimetry assays demonstrate that phosphorylation of Rb reduces the affinity of E2FTD binding ~250-fold and that phosphorylation at S608/S612 and T356/T373 is necessary and sufficient for this effect. A nuclear magnetic resonance (NMR) assay identifies phosphorylation-driven conformational changes in Rb that directly inhibit E2FTD binding. We find that phosphorylation at S608/S612 promotes an intramolecular association between a conserved sequence in the flexible pocket linker and the pocket domain of Rb that occludes the E2FTD binding site. We also find that phosphorylation of Thr356/Thr373 inhibits E2FTD binding in a manner that requires the Rb N-terminal domain (RbN). Taken together, our results suggest two distinct mechanisms for how phosphorylation of Rb modulates association between E2FTD and the Rb pocket and describe for the first time a function for the structured N-terminal domain in Rb inactivation.

The Na-K-2Cl cotransporter (NKCC1) participates in epithelial transport and in cell volume maintenance by mediating the movement of ions and water across plasma membranes. Functional studies have previously demonstrated that NKCC1 activity is stimulated by protein phosphatase 1 (PP1) inhibitors. In this study, we utilized both in vivo (heterologous cRNA expression in Xenopus laevis oocytes), and in vitro ((32)P phosphorylation assays with GST-fusion proteins) experiments to determine if PP1 exerts its inhibitory effect directly on the cotransporter, or indirectly by affecting the activating kinase. We found that PP1 reduced NKCC1 activity in frog oocytes under both isotonic and hypertonic conditions to the same level as in water-injected controls. Interestingly, mutation of key residues in the PP1 binding motif located in the N-terminal tail of NKCC1 significantly reduced the inhibitory effect of PP1. In vitro experiments performed with recombinant PP1, SPAK (Ste20-related proline alanine-rich kinase which activates NKCC1), and the N-terminus of NKCC1 fused to GST demonstrated that PP1 dephosphorylated both the kinase and the cotransporter in a time-dependent manner. More importantly, PP1 dephosphorylation of SPAK was significantly greater when protein-protein interaction between the kinase and the N-terminal tail of NKCC1 was present in the reaction, indicating the necessity of scaffolding the phosphatase and kinase in proximity to one another. Taken together, our data are consistent with PP1 inhibiting NKCC1 activity directly by dephosphorylating the cotransporter and indirectly by dephosphorylating SPAK.

It has previously been shown that the acylphosphatase from Sulfolobus Solfataricus (Sso AcP) is capable of forming amyloid-like aggregates under conditions in which the native structure is maintained and via the transient formation of native-like aggregates. Based on the previously determined NMR structure of the native protein, showing a ferredoxin-like fold and the peculiar presence of an unstructured N-terminal segment, we show here, at a molecular level using NMR spectroscopy, that indeed Sso AcP remains in a native-like conformation when placed in aggregating conditions and that such a native-like structure persists when the protein forms the initial aggregates, at least within the low-molecular-weight species. The analysis carried out under different solution conditions, based on the measurement of the combined 1H and 15N chemical shifts and H/D exchange rates, enabled the most significant conformational changes to be monitored upon transfer of the monomeric state into aggregating conditions and upon formation of the initial native-like aggregates. Important increases of the H/D exchange rates throughout the native protein, accompanied by small and localised structural changes, in the monomeric protein were observed. The results also allow the identification of the intermolecular interaction regions within the native-like aggregates, that involve, in particular, the N-terminal unstructured segment, the apical region including strands S4 and S5 with the connecting loop, and the opposite active site.

Mechanisms controlling human multipotent mesenchymal (stromal) stem cell (hMSC) differentiation into osteoblasts or adipocytes are poorly understood. We have previously demonstrated that Wnt signaling in hMSC enhanced osteoblast differentiation and inhibited adipogenesis by comparing two hMSC cell lines that are overexpressing mutated forms of Wnt co-receptor LRP5: T253I (hMSC-LRP5T253) and T244M (hMSC-LRP5T244) conducting high and low level of Wnt signaling, respectively. To explore the underlying molecular mechanisms, we compared gene expression profiles of hMSC-LRP5T253 and hMSC-LRP5T244 treated with Wnt3a using whole-genome expression microarrays and found that TNFRSF19 is differentially up-regulated between the two cells lines. Bioinformatic analysis and dual luciferase assay of its promoter revealed that TNFRSF19 transcript 2 (TNFRSF19.2) is a target of canonical Wnt signaling. Knocking down TNFRSF19 in hMSC-LRP5T253 cells decreased Wnt3a-induced osteoblast differentiation marker alkaline phosphates (ALP) activity and its overexpression in hMSC-LRP5T244 cells increased ALP activity. In addition, TNFRSF19 was negatively regulated by adipogenic transcription factor CCAAT/enhancer binding proteins (C/EBP). Knocking down TNFRSF19 in hMSC-LRP5T253 cells or its overexpression in hMSC-LRP5T244 cells significantly increased or decreased adipogenesis, respectively. In conclusion, we revealed a novel function of TNFRSF19 as a factor mediating differentiation signals that determine hMSC differentiating fate into osteoblasts or adipocytes.

We have previously shown a novel link between human protease-activated receptor-1 (hPar)-1 and beta-catenin stabilization. While it is well recognized that Wnt signaling leads to beta-catenin accumulation, the role of PAR1 the process is unknown. We provide here evidence that PAR1 induces beta-catenin stabilization independent of Wnt, Frizzled (Fz) and the co-receptor low density lipoprotein related protein 5/6; LRP5/6 and identify selective mediators of PAR1-beta-catenin axis. Immunohistological analyses of hPar1-transgenic (tg) mouse mammary tissues show the expression of both Galpha12 and Galpha13 compared with age-matched control counterparts. However, only Galpha13 was found to be actively involved in PAR1-induced beta-catenin stabilization. Indeed, a dominant negative (DN) form of Galpha13 inhibited both PAR1-induced Matrigel invasion and Lef/Tcf (lymphoid enhancer factor/T cell factor) transcription activity. PAR1-Galpha13 association is followed by the recruitment of dishevelled (DVL), an upstream Wnt signaling protein via the DIX domain. siRNA-DVL silencing leads to a reduction in PAR1-induced Matrigel invasion, inhibition of Lef/Tcf transcription activity and decreased beta-catenin accumulation. It is of note that PAR1 also promotes the binding of beta-arrestin-2 to DVL, suggesting a role for beta-arrestin-2 in PAR1-induced DVL phosphorylation dynamics. While infection of siRNA-LRP5/6 or the use of the Wnt antagonists, soluble Frizzled related protein2; SFRP2 or SFRP5 potently reduce Wnt3A-mediated beta-catenin accumulation, no effect is observed on PAR1-induced beta-catenin stabilization. Collectively, our data show that PAR1 mediates beta-catenin stabilization independent of Wnt. We propose here a novel cascade of PAR1-induced Galpha13-DVL axis in cancer and beta-catenin stabilization.

Mitogen-Activated Protein Kinase-Kinase 3 (MAP2K3) is a member of the dual specificity kinase group. Growing evidence links MAP2K3 to invasion and tumor progression. Here, we identify MAP2K3 as a transcriptional target of endogenous gain-of-function p53 mutants R273H, R175H, and R280K. We show that MAP2K3 modulation occurred at the mRNA and protein levels and that endogenous mutp53 proteins are capable of binding to and activate the MAP2K3 promoter. In addition, we found that the p53 mutants studied regulate MAP2K3 gene expression through the involvement of the transcriptional co-factors NF-Y and NFkB. Finally, functional studies showed that endogenous MAP2K3 knockdown inhibits proliferation and survival of human tumor cells, whereas the ectopic expression of MAP2K3 can rescue the proliferative defect induced by mutp53 knockdown. Taken together, our findings define a novel player through which mutp53 exerts its gain-of-function activity in cancer cells.

ATP-binding cassette (ABC) transporters constitute a large class of molecular pumps whose central role in chemotherapy resistance has highlighted their clinical relevance. We investigated whether the lipid composition of the membrane affects the function and structure of HorA, a bacterial ABC multidrug transporter. When the transporter was reconstituted in a bilayer where phosphatidylethanolamine (PE), the main lipid of the bacterial membrane, was replaced with phosphatidylcholine (PC), ATP hydrolysis and substrate transport became uncoupled. Although ATPase activity was maintained, HorA lost its ability to extrude the prototypical substrate Hoechst33342. Attenuated Total Reflection-Fourier Transform Infrared spectroscopy (ATR-FTIR) revealed that, although the secondary structure of the protein was unaffected, the orientation of the transmembrane helices (TM) was modified by the change in lipid composition. The orientation of the backbone carbonyls indicated that the helices opened wider in PE vs. PC-containing liposomes, with a 10 degrees difference. This was supported by hydrogen/deuterium exchange studies showing increased protection of the backbone from the solvent in PC-containing liposomes. Electron Paramagnetic Resonance was used to further probe the structural change. In the PC-containing liposomes we observed increased mobility of the spin label in TM4, along with increased exposure to molecular oxygen, used as a hydrophobic quencher. This indicates that the lipid change induced modification of the orientation of TM4, exposing Cys180 to the lipid phase. The lipid composition of the bilayer thus modulates the structure of HorA, and in turn its ability to extrude its substrates.

Efficient proliferation of Mycobacterium tuberculosis (Mtb) inside macrophage requires that the essential response regulator MtrA becomes optimally phosphorylated. However, the genomic targets of MtrA have not been identified. We show by chromatin immunoprecipitation and DNase I footprinting that the chromosomal origin of replication, oriC, and the promoter for the major secreted immunodominant antigen Ag85B, encoded by fbpB, are MtrA targets. DNAse I footprinting analysis revealed that MtrA recognizes two direct repeats of GTCACAgcg-like sequences and that MtrA~P, the phosphorylated form of MtrA, binds preferentially to these targets. The oriC contains several MtrA motifs, and replacement of all motifs or of a single select motif with TATATA compromises the ability of oriC plasmids to maintain stable autonomous replication in wild type and MtrA overproducing strains, indicating that the integrity of the MtrA motif is necessary for oriC replication. The expression of the fbpB gene is found to be down-regulated in Mtb cells upon infection when these cells overproduce WT MtrA but not when they overproduce a non-phosphorylated MtrA, indicating that MtrA~P regulates fbpB expression. We propose that MtrA is a regulator of oriC replication and that the MtrA's to affect apparently unrelated targets, i.e. oriC and fbpB, reflect its main role as a coordinator between Mtb's proliferative and its pathogenic functions.

The PI3K-PKB/Akt/PTEN signal transduction pathway orchestrates a variety of fundamental cell processes and its deregulation is implicated in many human diseases. While the importance of this pathway to many cellular functions is well established, the mechanisms by which it achieves context-specific physiological outcomes in response to a variety of stimuli, using a relatively limited pool of effectors, remain largely unknown. Spatial restriction of signaling events is one mean by which cells coordinate specific responses using common molecules. To investigate the subcellular location-specific roles of the major PI3K effector PKB/Akt in various cell processes, we have developed a novel experimental system employing cellular compartment-directed PKB/Akt pseudo-substrate inhibitors. Subcellular location-restricted PKB/Akt inhibition in the 3T3L1 adipocyte differentiation model revealed that nuclear and plasma membrane, but not cytoplasmic, PKB/Akt activity is required for terminal adipocyte differentiation. Nuclear and plasma membrane pools of PKB/Akt were found to contribute to distinct stages of adipocyte differentiation, revealing that PKB/Akt activity impacts multiple points of this program. Our work establishes the use of localized pseudosubstrate PKB/Akt inhibitors as an effective method for the dissection of PKB/Akt signaling.

The acGFPL is the first-identified member of a novel, colorless and non-fluorescent group of GFP-like proteins. Its mutant aceGFP, with Gly replacing the invariant catalytic Glu222, demonstrates relatively fast maturation rate and bright green fluorescence (lambdaex = 480 nm, lambdaem = 505 nm). The reverse Gly222Glu single mutation in aceGFP results in the immature, colorless variant aceGFP-G222E, which undergoes irreversible photoconversion to a green fluorescent state under UV light exposure. Here we present a high-resolution crystallographic study of aceGFP and aceGFP-G222E in the immature and UV photoconverted states. A unique and striking feature of the colorless aceGFP-G222E structure is the chromophore in the trapped intermediate state, where cyclization of the protein backbone has occurred, but Tyr66 still stays in the native, non-oxidized form, with Calpha and Cbeta atoms in the sp3 hybridization. This experimentally observed immature aceGFP-G222E structure, characterized by the non-coplanar arrangement of the imidazolone and phenolic rings, has been attributed to one of the intermediate states in the GFP chromophore biosynthesis. The UV irradiation (lambda = 250 - 300 nm) of aceGFP-G222E drives the chromophore maturation further to a green fluorescent state, characterized by the conventional coplanar bicyclic structure with the oxidized double Tyr66 Calpha-Cbeta bond and the conjugated system of pi-electrons. Structure-based site-directed mutagenesis has revealed a critical role of the proximal Tyr220 in the observed effects. In particular, an alternative reaction pathway via Tyr220 rather than conventional wt Glu222 has been proposed for aceGFP maturation.

BubR1 is essential for the mitotic checkpoint which prevents aneuploidy in cellular progeny by triggering anaphase delay in response to kinetochores incorrectly/not attached to the mitotic spindle. Here we define the molecular architecture of the functionally significant N-terminal region of human BubR1 and present the 1.8 A crystal structure of its tetratricopeptide repeat (TPR) domain. The structure reveals divergence from the classical TPR fold and is highly similar to the TPR domain of budding yeast Bub1. Shared distinctive features include a disordered loop insertion, a 3(10)-helix, a tight turn involving glycine positive varphi angles, and non-canonical packing of and between TPR motifs. We also define the molecular determinants of the interaction between BubR1 and kinetochore protein Blinkin. We identify a shallow groove on the concave surface of the BubR1 TPR domain that forms multiple discrete and potentially cooperative interactions with Blinkin. Finally, we present evidence for a direct interaction between BubR1 and Bub1 mediated by regions C-terminal to their TPR domains. This interaction provides a mechanism for Bub1-dependent kinetochore recruitment of BubR1. We thus provide novel molecular insights into the structure of BubR1 and its interactions at the kinetochore-microtubule interface. Our studies pave the way for future structure-directed engineering aimed at dissecting the roles of kinetochore-bound and other pools of BubR1 in vivo.

Structural characterization of glutamate cysteine ligase (GCL), the enzyme that catalyzes the initial, rate-limiting step in glutathione biosynthesis, has revealed many of the molecular details of substrate recognition. To further delineate the mechanistic details of this critical enzyme, we have determined the structures of two inhibited forms of S. cerevisiae GCL (ScGCL), which shares significant sequence identity with the human enzyme. In vivo, GCL activity is feedback regulated by glutathione. Examination of the structure of ScGCL-glutathione complex (2.5 A; R=19.9%, Rfree=25.1%) indicates that the inhibitor occupies both the glutamate- and the presumed cysteine-binding site, and disrupts the previously observed Mg2+ coordination in the ATP binding site. L-buthionine-S-sulfoximine (BSO) is a mechanism-based inhibitor of GCL and has been used extensively to deplete glutathione in cell culture and in vivo model systems. Inspection of the ScGCL-BSO structure (2.2 A; R=18.1%, Rfree=23.9%) confirms that BSO is phosphorylated on the sulfoximine nitrogen to generate the inhibitory species and reveals contacts that likely contribute to transition state stabilization. Overall, these structures advance our understanding of the molecular regulation of this critical enzyme and provide additional details of the catalytic mechanism of the enzyme.

In addition to its endocytic function, the LDL receptor-related protein 1 (LRP1) also contributes to cell signaling events. In the current study, the potential of LRP1 to modulate the PDGF signaling pathway was investigated. PDGF is a key regulator of cell migration and proliferation and mediates the tyrosine phosphorylation of LRP1 within its cytoplasmic domain. In WI-38 fibroblasts, PDGF-mediated LRP1 tyrosine phosphorylation occurred at 37 oC, but not at 4 oC where endocytosis is minimized. Further, blockade of endocytosis with the dynamin inhibitor, dynasore, also prevented PDGF-mediated LRP1 tyrosine phosphorylation. Immunofluorescence studies revealed co-localization of LRP1 with the PDGFR following PDGF treatment within endosomal compartments, while surface biotinylation experiments confirmed that phosphorylated LRP1 primarily originates from intracellular compartments. Together, the data reveal association of these two receptors in endosomal compartments where they form a signaling complex. To study the contribution of LRP1 to PDGF signaling we used mouse embryonic fibroblasts genetically deficient in LRP1 and identified phenotypic changes in these cell lines in response to PDGF stimulation by performing phospho-site profiling. Of 38 phosphorylated proteins analyzed, 8 were significantly different in LRP1 deficient fibroblasts, and were restored when LRP1 was expressed back in these cells. Importantly, the results revealed that LRP1 expression is necessary for PDGF-mediated activation of ERK. Overall, the studies reveal that LRP1 associates with the PDGFR in endosomal compartments and modulates its signaling properties affecting the MAPK and Akt/PI3K pathways.