Fluorescence resonance energy transfer (FRET) and proximity ligation assays reveal functionally relevant homo- and heteromeric complexes among hyaluronan synthases HAS1, HAS2, and HAS3

In vertebrates, hyaluronan is produced in the plasma membrane from cytosolic UDP-sugar substrates by hyaluronan synthase 1-3 (HAS1-3) isoenzymes that transfer N-acetylglucosamine (GlcNAc) and glucuronic acid (GlcUA) in alternative positions in the growing polysaccharide chain during its simultaneous extrusion into the extracellular space. It has been shown that HAS2 immunoprecipitates contain functional HAS2 homomers and also heteromers with HAS3 (Karousou, E., Kamiryo, M., Skandalis, S. S., Ruusala, A., Asteriou, T., Passi, A., Yamashita, H., Hellman, U., Heldin, C. H., and Heldin, P. (2010) The activity of hyaluronan synthase 2 is regulated by dimerization and ubiquitination. J. Biol. Chem. 285, 23647-23654). Here we have systematically screened in live cells, potential interactions among the HAS isoenzymes using fluorescence resonance energy transfer (FRET) and flow cytometric quantification. We show that all HAS isoenzymes form homomeric and also heteromeric complexes with each other. The same complexes were detected both in Golgi apparatus and plasma membrane by using FRET microscopy and the acceptor photobleaching method. Proximity ligation assays with HAS antibodies confirmed the presence of HAS1-HAS2, HAS2-HAS2, and HAS2-HAS3 complexes between endogenously expressed HASs. C-terminal deletions revealed that the enzymes interact mainly via uncharacterized N-terminal 86-amino acid domain(s), but additional binding site(s) probably exist in their C-terminal parts. Of all the homomeric complexes HAS1 had the lowest and HAS3 the highest synthetic activity. Interestingly, HAS1 transfection reduced the synthesis of hyaluronan obtained by HAS2 and HAS3, suggesting functional cooperation between the isoenzymes. These data indicate a general tendency of HAS isoenzymes to form both homomeric and heteromeric complexes with potentially important functional consequences on hyaluronan synthesis.

RhoD regulates cytoskeletal dynamics via the actin nucleation-promoting factor WASp homologue associated with actin Golgi membranes and microtubules

RhoD has a role in actin dynamics that is distinct from RhoA, Rac1, and Cdc42. Data presented here indicate that RhoD binds the actin nucleation–promoting factor WASp homologue associated with actin Golgi membranes and microtubules (WHAMM) and the related filamin A–binding protein FILIP1. WHAMM acts downstream of RhoD, and both proteins coordinate vital cellular processes, such as actin dynamics, cell attachment, and cell migration.

The Rho GTPases have mainly been studied in association with their roles in the regulation of actin filament organization. These studies have shown that the Rho GTPases are essential for basic cellular processes, such as cell migration, contraction, and division. In this paper, we report that RhoD has a role in the organization of actin dynamics that is distinct from the roles of the better-studied Rho members Cdc42, RhoA, and Rac1. We found that RhoD binds the actin nucleation–promoting factor WASp homologue associated with actin Golgi membranes and microtubules (WHAMM), as well as the related filamin A–binding protein FILIP1. Of these two RhoD-binding proteins, WHAMM was found to bind to the Arp2/3 complex, while FILIP1 bound filamin A. WHAMM was found to act downstream of RhoD in regulating cytoskeletal dynamics. In addition, cells treated with small interfering RNAs for RhoD and WHAMM showed increased cell attachment and decreased cell migration. These major effects on cytoskeletal dynamics indicate that RhoD and its effectors control vital cytoskeleton-driven cellular processes. In agreement with this notion, our data suggest that RhoD coordinates Arp2/3-dependent and FLNa-dependent mechanisms to control the actin filament system, cell adhesion, and cell migration.

The activity of hyaluronan synthase 2 is regulated by dimerization and ubiquitination

Hyaluronan is a component of the extracellular matrix, which affects tissue homeostasis. In this study, we investigated the regulatory mechanisms of one of the hyaluronan-synthesizing enzymes, HAS2. Ectopic expression of Flag- and 6myc-HAS2 in COS-1 cells followed by immunoprecipitation and immunoblotting revealed homodimers; after co-transfection with Flag-HAS3, also heterodimers were seen. Furthermore, the expressed HAS2 was ubiquitinated. We identified one acceptor site for ubiquitin on lysine residue 190. Mutation of this residue led to inactivation of the enzymatic activity of HAS2. Interestingly, K190R-mutated HAS2 formed dimers with wt HAS2 and quenched the activity of wt HAS2, thus demonstrating a functional role of the dimeric configuration.

Nck adapters are involved in the formation of dorsal ruffles, cell migration, and Rho signaling downstream of the platelet-derived growth factor beta receptor

The SH3 and SH2 domain-containing adapter proteins Nck1 and Nck2 are known to function downstream of activated tyrosine kinase receptors, such as the platelet-derived growth factor (PDGF) receptors. The SH2 domain of Nck1 binds to phosphorylated tyrosine residue 751 in PDGFbeta receptor and has been suggested to have a role in the PDGF-induced mobilization of the actin filament system. Because Tyr-751 is a site for additional receptor interactors, it has been difficult to discriminate the signaling from Nck from signaling via other molecules. For this reason we have used mouse embryonic fibroblasts derived from mice in which the genes for Nck1 and Nck2 have been inactivated by gene targeting (knock-out (KO) cells). The mutant cells had a reduced ability to form edge ruffles in response to PDGF, and the presence of Nck was obligatory for the formation of dorsal ruffles. In addition, the KO cells had a reduced chemotactic and migratory potential. Importantly, KO cells had reduced cell attachment properties and a reduced ability to form focal adhesions in response to serum stimulation. Moreover, signaling involving the Rho GTPases was defective in KO cells. In summary, our observations suggest that the Nck adapters are needed for signaling to Rho GTPases and actin dynamics downstream of the PDGFbeta receptor.

Taking Rho GTPases to the next level: the cellular functions of atypical Rho GTPases

The Rho GTPases are influential regulators of signalling pathways that control vital cellular processes such as cytoskeletal dynamics, gene transcription, cell cycle progression and cell transformation. A vast majority of the studies involving Rho GTPases have been focused to the famous triad, Cdc42, Rac1 and RhoA, but this protein family actually harbours 20 members. Recently, the less known Rho GTPases have received increased attention. Many of the less studied Rho GTPases have structural, as well as, functional features which makes it pertinent to classify them as atypical Rho GTPases. This review article will focus on the critical aspects of the atypical Rho GTPases, RhoH, Wrch-1, Chp and RhoBTB. These proteins are involved in a broad spectre of biological processes, such as cytoskeletal dynamics, T-cell signalling and protein ubiquitinylation. We will also discuss the roles of atypical Rho GTPases as oncogenes or tumour suppressors, as well as their potential involvement in human diseases.

The atypical Rho GTPases Miro-1 and Miro-2 have essential roles in mitochondrial trafficking

We recently described the atypical Rho GTPases Miro-1 and Miro-2. These proteins have tandem GTP-binding domains separated by a linker region with putative calcium-binding motives. In addition, the Miro GTPases have a C-terminal transmembrane domain, which confers targeting to the mitochondria. It was reported previously that a constitutively active mutant of Miro-1 induced a clustering of the mitochondria. This response can be separated into two distinct phenotypes: a formation of aggregated mitochondria and the appearance of thread-like mitochondria probably caused by defects in mitochondrial trafficking. The first GTPase domain is required for the clustering of the mitochondria, but the effect is not dependent on the EF-hands. Miro-2 only induces aggregation and not the formation of thread-like mitochondria. Moreover, we show that Miro interacts with the Kinesin-binding proteins, GRIF-1 and OIP106, suggesting that the Miro GTPases form a link between the mitochondria and the trafficking apparatus of the microtubules.

Isolation and characterisation of DOCK8, a member of the DOCK180-related regulators of cell morphology

In a yeast two-hybrid system screen for Cdc42-interacting proteins, we identified a protein with similarity to the CrkII-binding protein DOCK180. A cDNA clone of this protein, designated DOCK8, encoded a gene-product of 1701 amino acid residues with a molecular mass of 190 kDa. Immunofluorescence staining showed that transiently transfected HA-tagged DOCK8, as well as endogenous DOCK8, was present at the cell edges in areas undergoing lamellipodia formation. Transient transfection of a C-terminal fragment of DOCK8 resulted in the formation of vesicular structures. Interestingly, these vesicles also contained filamentous actin. These data suggest an involvement of DOCK8 in processes that affect the organisation of filamentous actin.

Atypical Rho GTPases have roles in mitochondrial homeostasis and apoptosis

The human genomic sequencing effort has revealed the presence of a large number of Rho GTPases encoded by the human genome. Here we report the characterization of a new family of Rho GTPases with atypical features. These proteins, which were called Miro-1 and Miro-2 (for mitochondrial Rho), have tandem GTP-binding domains separated by a linker region containing putative calcium-binding EF hand motifs. Genes encoding Miro-like proteins were found in several eukaryotic organisms from Saccharomyces cerevisiae, Caenorhabditis elegans, and Drosophila melanogaster to mammals, indicating that these genes evolved early during evolution. Immunolocalization experiments, in which transfected NIH3T3 and COS 7 cells were stained for ectopically expressed Miro as well as for the endogenous Miro-1 protein, showed that Miro was present in mitochondria. Interestingly, overexpression of a constitutively active mutant of Miro-1 (Miro-1/Val-13) induced an aggregation of the mitochondrial network and resulted in an increased apoptotic rate of the cells expressing activated Miro-1. These data indicate a novel role for Rho-like GTPases in mitochondrial homeostasis and apoptosis.

MEK is a negative regulator of Stat5b in PDGF-stimulated cells

In this study we show that platelet-derived growth factor (PDGF)-induced DNA binding as well as transcriptional activation of Stat5b are markedly increased by inhibition of the MAP (mitogen-activated protein) kinase kinase MEK. In addition to the previously demonstrated tyrosine phosphorylation, we show that serine and threonine phosphorylation of Stat5b is increased in response to PDGF stimulation. However, inhibition of MEK had no effect on the phosphorylation level of Stat5b or on the nuclear translocation of Stat5b. These observations indicate that MEK is a negative modulator of PDGF-induced Stat5b activation through a mechanism not involving direct phosphorylation of Stat5b.

Platelet-derived growth factor (PDGF)-induced actin rearrangement is deregulated in cells expressing a mutant Y778F PDGF beta-receptor

Platelet-derived growth factor-stimulated actin rearrangement and edge ruffle formation have previously been shown to be dependent on activation of phosphatidylinositol 3′-kinase, the activity of which also is important for directed migration of cells. This lipid kinase binds to phosphorylated tyrosine residues Y740 and Y751 in the kinase insert of the human platelet-derived growth factor ss-receptor. We examined the role of two other tyrosine residues in the kinase insert of this receptor, Y775 and Y778, for ligand-induced actin rearrangement. Both were shown to be phosphorylation sites; Y775 was only marginally phosphorylated in cells expressing the wild-type ss-receptor, whereas Y778 was phosphorylated at higher stoichiometry. Mutant receptors Y775F, Y778F and Y775/778F were active kinases and mediated proliferative responses when expressed in porcine aortic endothelial cells. Fluorescence staining of actin in platelet-derived growth factor-stimulated PAE cells revealed that Y778 is involved in regulation of the actin cytoskeleton since the cells contained, apart from edge ruffles and circular ruffles, a novel type of giant ruffle on the dorsal side of the cell, which consisted of irregular multilayered actin structures. Mutation at Y778 had no effect on activation of phosphatidylinositol 3′-kinase, nor on the GTPase activating protein of Ras and phospholipase C(gamma), and the extent of directed migration towards platelet-derived growth factor of these cells was not changed. We conclude that actin rearrangement is regulated in part by Y778 in the platelet-derived growth factor ss-receptor, potentially through binding of a novel signaling molecule to this site.

Coexpression of the membrane glycoproteins G1 and G2 of Hantaan virus is required for targeting to the Golgi complex

To study the intracellular transport and targeting to the Golgi complex of the membrane glycoproteins G1 and G2 of Hantaan virus, we have expressed them together and separately using recombinant vaccinia viruses. When expressed from the same recombinant vaccinia virus, G1 and G2 were localized to the Golgi complex as analyzed by both immunofluorescence and subcellular fractionation. However, when the glycoproteins were expressed from separate recombinant viruses, both proteins remained in the endoplasmic reticulum. Using several monoclonal antibodies, it was found that G1 expressed alone did not acquire its correct conformation. Finally, if cells were coinfected with G1- and G2-expressing recombinant viruses, the proteins were again targeted to the Golgi complex. The N-linked glycans remained in all cases largely endoglycosidase-H sensitive. With none of the recombinant viruses were expression of the glycoproteins observed on the cell surface. Neither did chasing in the presence of cycloheximide result in the surface expression of G1 or G2. Our results indicate that for transport out of the endoplasmic reticulum and proper targeting to the Golgi complex, the two glycoproteins have to be coexpressed. The most likely interpretation is that G1 and G2 have to interact with each other in the endoplasmic reticulum in order to become transport competent.

Dissociation and reassociation of oligomeric viral glycoprotein subunits in the endoplasmic reticulum

The vesicular stomatitis virus (VSV) glycoprotein (G) forms noncovalently linked trimers in the endoplasmic reticulum (ER) prior to transport to the cell surface. Here we examined the formation of heterotrimers between wild-type and mutant subunits that were retained in the ER by C-terminal retention signals. When G protein was coexpressed with mutant subunits that formed trimers at the wild-type rate and were transported from the ER at the wild-type rate, heterotrimers were readily detected. In contrast, when G protein was coexpressed with mutant subunits that formed trimers at the wild-type rate, but were retained in the ER, heterotrimers were not detected unless transport of the wild-type molecules from the ER was blocked. After removal of transport block, the heterotrimers then dissociated and reassorted to homotrimers of the mutant protein that were retained in the ER and wild-type trimers that were transported to the cell surface. These and other results presented here indicate that there is an equilibrium between G protein trimers and monomers in vivo, at least in the ER. This equilibrium may function to allow escape of wild-type subunits from aberrant retained subunits.

Oligomerization of glycolipid-anchored and soluble forms of the vesicular stomatitis virus glycoprotein

The vesicular stomatitis virus glycoprotein forms noncovalently linked trimers in the endoplasmic reticulum before being transported to the Golgi apparatus. The experiments reported here were designed to determine if the extracellular domain of the glycoprotein contains structural information sufficient to direct trimer formation. To accomplish this, we generated a construct encoding G protein with the normal transmembrane and anchor sequences replaced with the sequence encoding 53 C-terminal amino acids from the Thy-1.1 glycoprotein. We show here that these sequences were able to specify glycolipid addition to the truncated G protein, probably after cleavage of 31 amino acids derived from Thy-1.1. The glycolipid-anchored G protein formed trimers and was expressed on the cell surface in a form that could be cleaved by phosphoinositol-specific phospholipase C. However, the rate of transport was reduced, compared with that of wild-type G protein. A second form of the G protein was generated by deletion of only the transmembrane and cytoplasmic domains. This mutant protein also formed trimers with relatively high efficiency and was secreted slowly from cells.

Differential effects of mutations in three domains on folding, quaternary structure, and intracellular transport of vesicular stomatitis virus G protein

The vesicular stomatitis virus glycoprotein (G protein) is an integral membrane protein which assembles into noncovalently associated trimers before transport from the endoplasmic reticulum. In this study we have examined the folding and oligomeric assembly of twelve mutant G proteins with alterations in the cytoplasmic, transmembrane, or ectodomains. Through the use of conformation-specific antibodies, we found that newly synthesized G protein folded into a conformation similar to the mature form within 1-3 min of synthesis and before trimer formation. Mutant proteins not capable of undergoing correct initial folding did not trimerize, were not transported, and were found in large aggregates. They had, as a rule, mutations in the ectodomain, including several with altered glycosylation patterns. In contrast, mutations in the cytoplasmic domain generally had little effect on folding and trimerization. These mutant proteins, whose ectodomains were identical to the wild-type by several assays, were either transported to the cell surface slowly or not at all. We concluded that while correct ectodomain folding and trimer formation are prerequisites for transport, they alone are not sufficient. The results suggest that the cytoplasmic domain of the wild-type protein may facilitate rapid, efficient transport from the ER, which can be easily affected or eliminated by tail mutations that do not detectably affect the ectodomain.

Effects of altered cytoplasmic domains on transport of the vesicular stomatitis virus glycoprotein are transferable to other proteins

Alterations of the cytoplasmic domain of the vesicular stomatitis virus glycoprotein (G protein) were shown previously to affect transport of the protein from the endoplasmic reticulum, and recent studies have shown that this occurs without detectable effects on G protein folding and trimerization (R. W. Doms et al., J. Cell Biol., in press). Deletions within this domain slowed exit of the mutant proteins from the endoplasmic reticulum, and replacement of this domain with a foreign 12-amino-acid sequence blocked all transport out of the endoplasmic reticulum. To extend these studies, we determined whether such effects of cytoplasmic domain changes were transferable to other proteins. Three different assays showed that the effects of the mutations on transport of two membrane-anchored secretory proteins were the same as those observed with vesicular stomatitis virus G protein. In addition, possible effects on oligomerization were examined for both transported and nontransported forms of membrane-anchored human chorionic gonadotropin-alpha. These membrane-anchored forms, like the nonanchored human chorionic gonadotropin-alpha, had sedimentation coefficients consistent with a monomeric structure. Taken together, our results provide strong evidence that these cytoplasmic mutations affect transport by affecting interactions at or near the cytoplasmic side of the membrane.