Lipoxygenase enzymes initiate diverse signaling pathways by specifically directing oxygen to different carbons of arachidonate and other polyunsaturated acyl chains, but structural origins of this specificity have remained unclear. We therefore determined the nature of the lipoxygenase interaction with the polar-end of a paramagnetic lipid by electron paramagnetic resonance spectroscopy. Distances between selected grid points on soybean seed lipoxygenase-1 (SBL1) and a lysolecithin spin-labeled on choline were measured by pulsed (electron) dipolar spectroscopy. The protein grid was designed by structure-based modeling so that five natural side chains were replaced with spin labels. Pairwise distances in 10 doubly spin-labeled mutants were examined by pulsed dipolar spectroscopy, and a fit to the model was optimized. Finally, experimental distances between the lysolecithin spin and each single spin site on SBL1 were also obtained. With these 15 distances, distance geometry localized the polar-end and the spin of the lysolecithin to the region between the two domains in the SBL1 structure, nearest to E236, K260, Q264, and Q544. Mutation of a nearby residue, E256A, relieved the high pH requirement for enzyme activity of SBL1 and allowed lipid binding at pH 7.2. This general approach could be used to locate other flexible molecules in macromolecular complexes.

As obligate intracellular parasites, viruses exploit diverse cellular signaling machineries, including the mitogen-activated protein-kinase pathway, during their infections. We have demonstrated previously that the open reading frame 45 (ORF45) of Kaposi sarcoma-associated herpesvirus interacts with p90 ribosomal S6 kinases (RSKs) and strongly stimulates their kinase activities (Kuang, E., Tang, Q., Maul, G. G., and Zhu, F. (2008) J. Virol. 82, 1838 -1850). Here, we define the mechanism by which ORF45 activates RSKs. We demonstrated that binding of ORF45 to RSK increases the association of extracellular signalregulated kinase (ERK) with RSK, such that ORF45, RSK, and ERK formed high molecular mass protein complexes. We further demonstrated that the complexes shielded active pERK and pRSK from dephosphorylation. As a result, the complex-associated RSK and ERK were activated and sustained at high levels. Finally, we provide evidence that this mechanism contributes to the sustained activation of ERK and RSK in Kaposi sarcomaassociated herpesvirus lytic replication.The extracellular signal-regulated kinase (ERK) 2 mitogenactivated protein kinase (MAPK) signaling pathway has been implicated in diverse cellular physiological processes including proliferation, survival, growth, differentiation, and motility (1-4) and is also exploited by a variety of viruses such as Kaposi sarcoma-associated herpesvirus (KSHV), human cytomegalovirus, human immunodeficiency virus, respiratory syncytial virus, hepatitis B virus, coxsackie, vaccinia, coronavirus, and influenza virus (5-17). The MAPK kinases relay the extracellular signaling through sequential phosphorylation to an array of cytoplasmic and nuclear substrates to elicit specific responses (1, 2, 18). Phosphorylation of MAPK is reversible. The kinetics of deactivation or duration of signaling dictates diverse biological outcomes (19,20). For example, sustained but not transient activation of ERK signaling induces the differentiation of PC12 cells into sympathetic-like neurons and transformation of NIH3T3 cells (20 -22). During viral infection, a unique biphasic ERK activation has been observed for some viruses (an early transient activation triggered by viral binding or entry and a late sustained activation correlated with viral gene expression), but the responsible viral factors and underlying mechanism for the sustained ERK activation remain largely unknown (5,8,13,23).The p90 ribosomal S6 kinases (RSKs) are a family of serine/ threonine kinases that lie at the terminus of the ERK pathway (1, 24 -26). In mammals, four isoforms are known, RSK1 to RSK4. Each one has two catalytically functional kinase domains, the N-terminal kinase domain (NTKD) and C-terminal kinase domain (CTKD) as well as a linker region between the two. The NTKD is responsible for phosphorylation of exogenous substrates, and the CTKD and linker region regulate RSK activation (1,24,25). In quiescent cells ERK binds to the docking site in the C terminus of RSK (27-29). Upon mitogen stimulation, ERK is activ...

Open reading frame 45 (ORF45) of Kaposi's sarcoma-associated herpesvirus (KSHV) is an immediate-early and tegument protein that plays critical roles in antagonizing host antiviral responses. We have previously shown (Zhu et al, Proc. Natl. Acad. Sci. U. S. A., 99:5573–5578, 2002) that ORF45 suppresses activation of interferon regulatory factor 7 (IRF7), a crucial regulator of type I interferon gene expression, by blocking its virus-induced phosphorylation and nuclear accumulation. We report here further characterization of the mechanisms by which ORF45 inhibits IRF7 phosphorylation. In most cell types, IRF7 is phosphorylated and activated by IKKε and TBK1 after viral infection. We found that phosphorylation of IRF7 on Ser477 and Ser479 by IKKε or TBK1 is inhibited by ORF45. The inhibition is specific to IRF7 because phosphorylation of its close relative IRF3 is not affected by ORF45, implying that ORF45 does not inactivate the kinases directly. In fact, we found that ORF45 is phosphorylated efficiently on Ser41 and Ser162 by IKKε and TBK1. We demonstrated that ORF45 competes with the associated IRF7 and inhibits its phosphorylation by IKKε or TBK1 by acting as an alternative substrate.

The ORF45 protein of Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus-specific immediate-early tegument protein. Our previous studies have revealed its crucial roles in both early and late stages of KSHV infection. In this study, we surveyed the interactome of ORF45 using a panel of monoclonal antibodies. In addition to the previously identified extracellular regulated kinase (ERK) and p90 ribosomal S6 kinase (RSK) proteins, we found several other copurified proteins, including prominent ones of ϳ38 kDa and ϳ130 kDa. Mass spectrometry revealed that the 38-kDa protein is viral ORF33 and the 130-kDa protein is cellular USP7 (ubiquitin-specific protease 7). We mapped the ORF33-binding domain to the highly conserved carboxyl-terminal 19 amino acids (aa) of ORF45 and the USP7-binding domain to the reported consensus motif in the central region of ORF45. Using immunofluorescence staining, we observed colocalization of ORF45 with ORF33 or USP7 both under transfected conditions and in KSHV-infected cells. Moreover, we noticed ORF45-dependent relocalization of a portion of ORF33/USP7 from the nucleus to the cytoplasm. We found that ORF45 caused an increase in ORF33 protein accumulation that was abolished if either the ORF33-or USP7-binding domain in ORF45 was deleted. Furthermore, deletion of the conserved carboxyl terminus of ORF45 in the KSHV genome drastically reduced the level of ORF33 protein in KSHV-infected cells and abolished production of progeny virions. Collectively, our results not only reveal new components of the ORF45 interactome, but also demonstrate that the interactions among these proteins are crucial for KSHV lytic replication. IMPORTANCEKaposi's sarcoma-associated herpesvirus (KSHV) is the causative agent of several human cancers. KSHV ORF45 is a multifunctional protein that is required for KSHV lytic replication, but the exact mechanisms by which ORF45 performs its critical functions are unclear. Our previous studies revealed that all ORF45 protein in cells exists in high-molecular-weight complexes. We therefore sought to characterize the interactome of ORF45 to provide insights into its roles during lytic replication. Using a panel of monoclonal antibodies, we surveyed the ORF45 interactome in KSHV-infected cells. We identified two new binding partners of ORF45: the viral protein ORF33 and cellular ubiquitin-specific protease 7 (USP7). We further demonstrate that the interaction between ORF45 and ORF33 is crucial for the efficient production of KSHV viral particles, suggesting that the targeted interference with this interaction may represent a novel strategy to inhibit KSHV lytic replication. K aposi's sarcoma-associated herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma, the most common malignancy in HIV/AIDS patients. It is also associated with two lymphoproliferative disorders: primary effusion lymphoma and multicentric Castleman's disease (1-3). Like other herpesviruses, KSHV exhibits two alternative life cycles, a quiescent latent stage and a productiv...

Increased androgen receptor (AR) transcriptional activity mediated by coactivator proteins may drive castration resistant prostate cancer (CRPC) growth. Vav3, a Rho GTPase guanine nucleotide exchange factor (GEF), is over-expressed in human prostate cancers particularly in models of CRPC progression. Vav3 coactivates AR in a Vav3 pleckstrin homology (PH) domain-dependent but GEF-independent manner. Ectopic expression of Vav3 in androgen-dependent human prostate cancer cells conferred robust castration resistant xenograft tumor growth. Vav3 but not a Vav3 PH mutant greatly stimulated interaction between the AR amino and carboxyl termini (N-C interaction), which is required for maximal receptor transcriptional activity. Vav3 was distributed between the cytoplasm and nucleus with nuclear localization dependent upon the Vav3 PH domain. Membrane targeting of Vav3 abolished Vav3 potentiation of AR activity; whereas, nuclear targeting of a Vav3 PH mutant rescued AR coactivation suggesting that nuclear localization is a key function of the Vav3 PH domain. A nuclear role for Vav3 was further demonstrated by sequential chromatin immunoprecipitation assays, which revealed that Vav3 and AR were recruited to the same transcriptional complexes of an AR target gene enhancer. These data demonstrate the importance of Vav3 in CRPC and define a novel nuclear function of Vav3 in regulating AR activity.

The putative substrate-binding site in lipoxygenases is long and internal. There is little direct evidence about how the unsaturated fatty acid substrates enter and move within the cavity in order to position correctly for electron transfer reactions with the catalytic non-heme iron. An EPR spectroscopy approach, with spin labeled fatty acids, is taken here to investigate dynamic behavior of fatty acids bound to soybean lipoxygenase-1. The probes are labeled on carbons 5-, 8-, 10-, 12-and 16-of stearic acid. The EPR-determined affinity for the enzyme increases as the length of the alkyl end of the probe increases, with ΔΔG of −190 cal/methylene. The probes in the series exhibit similar enhanced paramagnetic relaxation by the iron center. These results indicate that the members of the series have a common binding site. All of the bound probes undergo considerable local mobility. The stearate spin labeled on carbon 5 has highest affinity for the lipoxygenase and it is a competitive inhibitor, with K i 9 μM. Surprisingly, this stearate labeled near the carboxyl end undergoes more local motion than those labeled in the middle of the chain, when it is bound. This shows that the carboxyl end of the fatty acid spin label is not rigidly docked on the protein. During catalysis, repositioning of the substrate carboxyl on the protein surface may be coupled to motion of portions of the chain undergoing reaction. KeywordsLipoxygenase; spin label; fatty acid; substrate motion; paramagnetic protein Lipoxygenase (LOX) enzymes catalyze the first step in one of the major pathways to molecules derived from arachidonic acid. A theme emerging from the medical perspective on different human lipoxygenases is one of positive and negative influences on human health from these molecules (1,2). Thus, there is much interest in being able to pharmaceutically modulate the activity of selected members of the lipoxygenase family while leaving other members functioning normally (3,4). Indeed, inhibitors selective for 5-, 12-or 15-arachidonic acid lipoxygenases have been developed (5-7). Lipoxygenases from both plant and animal sources have overall structural similarity with a long internal cavity that passes by the catalytic iron ion, based on available X-ray structures (8)(9)(10)(11)(12)(13)(14). Some determinants of substrate binding within this cavity have been examined by mutation and modeling (15)(16)(17)(18)(19)(20), but there is little direct * To whom correspondence should be addressed: Tel. (850) 644-8547. Fax: (850) 644-0481. E-mail: gaffney@bio.fsu.edu. 1 ABBREVIATIONS: cmc, critical micelle concentration; EPR, electron paramagnetic resonance; LOX, any lipoxygenase; SBL1, soybean lipoxygenase isoform-1; 13S-HPODE, 13S-hydroperoxyoctadecadienoic acid; cAOS, coral allene oxide synthase; doxyl-, 1-oxyl-2,2,5,5-tetramethyloxazolidinyl-; x-DSA, a doxyl stearic acid spin label with doxyl ring on carbon x; TEMPOL, 1-oxyl-2,2,6,6-tetramethyl-4-piperidinol; 2A zz , rigid limit separation of outer extrema in spin label EPR spectra; 2A ...

Coral allene oxide synthase (cAOS), a fusion protein with 8R-lipoxygenase in Plexaura homomalla, is a hemoprotein with sequence similarity to catalases. cAOS reacts rapidly with the oxidant peracetic acid to form heme compound I and intermediate II. Concomitantly, an electron paramagnetic resonance (EPR) signal with tyrosyl radical-like features, centered at a g-value of 2.004-2.005, is formed. The radical is identified as tyrosyl by changes in EPR spectra when deuterated tyrosine is incorporated in cAOS. The radical location in cAOS is determined by mutagenesis of Y193 and Y209. Upon oxidation, native cAOS and mutant Y209F exhibit the same radical spectrum, but no significant tyrosine radical forms in mutant Y193H, implicating Y193 as the radical site in native cAOS. Estimates of the side chain torsion angles for the radical at Y193, based on the β-proton isotropic EPR hyperfine splitting, A iso , are θ 1 = 21 to 30° and θ 2 = −99 to −90°. The results show that cAOS can cleave nonsubstrate hydroperoxides by a heterolytic path, although a homolytic course is likely taken in converting the normal substrate, 8R-hydroperoxyeicosatetraenoic acid (8R-HpETE), to product. Coral AOS achieves specificity for the allene oxide formed by selection of the homolytic pathway normally, while it inactivates by the heterolytic path with nonoptimal substrates. Accordingly, with the nonoptimal substrate, 13R-hydroperoxyoctadecadienoic acid (13R-HpODE), mutant Y193H is inactivated after turning over significantly fewer substrate molecules than required to inactivate native cAOS or the Y209F mutant because it cannot absorb oxidizing equivalents by forming a radical at Y193. Polyunsaturated fatty acids (PUFAs) 1 are rapidly converted to cell signaling molecules in multistep syntheses involving an initial lipoxygenase or prostaglandin synthase step, followed by further enzymatic conversions (1-3). These multistep syntheses are regulated by localization of the enzymes in subcellular compartments and by protein-protein associations. Examples include synthesis of leukotriene C 4 at the nuclear envelope of leukocytes (4) and synthesis of jasmonic acid by steps which are thought to begin in chloroplasts and to be completed in peroxisomes of green plant cells (5). A fusion protein identified in the coral Plexaura homomalla represents a third mode of regulation, in which an N-terminal catalaselike domain is fused to an 8R-lipoxygenase domain (6). The N-terminal domain (44 kDa) has allene oxide synthase (cAOS) activity with the 8R-hydroperoxide of arachidonic acid (8R- † This research was supported by National Institutes of Health Grant GM36232 (to B.J.G.).* To whom correspondence should be addressed. Phone: (850) 644-8547. Fax: (850) 644-8547. E-mail: gaffney@bio.fsu.edu.. ‡ These authors contributed equally.1 Abbreviations: cAOS, coral allene oxide synthase; BLC, bovine liver catalase; BSA, bovine serum albumin; CYP, cytochrome P450; DES, divinyl ether synthase; EPR, electron paramagnetic resonance; 8R-HpETE, 8R-hydroperoxy-9E,11Z-ei...

Background:The Rho GTPase guanine nucleotide exchange factor, Vav3, is overexpressed in human prostate cancer and enhances androgen receptor transcriptional activity. Results: Cdc37 is a novel Vav3 binding partner that enhances androgen receptor co-activation by Vav3 and increases prostate cancer cell proliferation. Conclusion: Vav3-Cdc37 interaction is required for maximal androgen receptor function and prostate cancer growth. Significance: Vav3-Cdc37 interaction is a potential therapeutic target for prostate cancer.