The protooncogene TCL1 is an Akt kinase coactivator

Human T cell prolymphocytic leukemia can result from chromosomal translocations involving 14q32.1 or Xq28 regions. The regions encode a family of protooncogenes (TCL1, MTCP1, and TCL1b) of unknown function. In yeast two-hybrid screening, we found that TCL1 interacts with Akt. All TCL1 isoforms bind to the Akt pleckstrin homology domain. Both in vitro and in vivo TCL1 increases Akt kinase activity and as a consequence enhances substrate phosphorylation. In vivo, TCL1 stabilizes the mitochondrial transmembrane potential and enhances cell proliferation and survival. In vivo, TCL1 forms trimers, which associate with Akt. TCL1 facilitates the oligomerization and activation of Akt. Our data show that TCL1 is a novel Akt kinase coactivator, which promotes Akt-induced cell survival and proliferation.     

Differential regulation of Akt kinase isoforms by the members of the TCL1 oncogene family.

The members of the TCL1 proto-oncogene family (TCL1, MTCP1, and TCL1b) bind to Akt1, increasing its phosphorylation status and kinase activity. This is thought to be secondary to the formation of TCL1-Akt oligomers within which Akt is preferentially phosphorylated. Here we show that, in contrast to Akt1 and Akt2, which bind to all members of the TCL1 family, Akt3 specifically interacts with TCL1 but not with MTCP1 or TCL1b. This association is functional, as the presence of TCL1 but not MTCP1 or TCL1b increased Akt3 kinase activity in in vitro kinase assays. Functional specificity is determined by the Akt pleckstrin homology domain as chimeric Akt1, where Akt1 PH domain was replaced by that of Akt3 was no longer able to interact with MTCP1 or TCL1b and its kinase activity was solely enhanced by TCL1. Moreover, we show that, in TCL1-overexpressing SUPT-11 T-cell leukemia and P3HR-1 Burkitt's lymphoma cell lines, TCL1 interacts with endogenous Akt1, Akt2, and Akt3. TCL1 enhanced hetero-oligomerization of Akt1 with Akt3 and as a consequence facilitated transphosphorylation of Akt molecules, which may contribute to Akt activation and TCL1-induced leukemogenesis in vivo.     

Inhibition of Akt kinase activity by a peptide spanning the betaA strand of the proto-oncogene TCL1

Akt plays a central role in the regulation of cellular anti-apoptosis underlying various human neoplastic diseases. We have demonstrated previously that TCL1 (a proto-oncogene underlying human T cell prolymphocytic leukemia) interacts with Akt and functions as an Akt kinase co-activator. With the aim to develop an Akt kinase inhibitor, we hypothesized that a peptide, which spans the Akt-binding site, binds to Akt and modulates Akt kinase activity and its downstream biological responses. Indeed, we demonstrated that a peptide, named "Akt-in" (Akt inhibitor, NH(2)-AVTDHPDRLWAWEKF-COOH, encompassing the betaA strand of human TCL1), interacted with Akt and specifically inhibited its kinase activity. Nuclear magnetic resonance studies suggested that interaction of Akt-in with the pleckstrin homology domain (PH) of Akt caused conformational changes on the variable loop 1 of Akt, the locus mediating phosphoinositide binding. Consistently, interaction of Akt-in with the Akt PH domain prevented phosphoinositide binding and hence inhibited membrane translocation and activation of Akt. Moreover, Akt-in inhibited not only cellular proliferation and anti-apoptosis in vitro but also in vivo tumor growth without any adverse effect. The roles of Akt, which possesses a PH domain, in intracellular signaling were well established. Hence, Akt inhibitors create an attractive target for anticancer therapy. However, no effective inhibitors specific for Akt have been developed. Akt-in, which inhibits association of phosphatidylinositol with Akt, is the first molecule to demonstrate specific Akt kinase inhibition potency. This observation will facilitate the design of specific inhibitors for Akt, a core intracellular survival factor underlying various human neoplastic diseases.     

Identification of oligomerization and drug-binding domains of the membrane fusion protein EmrA

Many pathogenic Gram-negative bacteria possess tripartite transporters that catalyze drug extrusion across the inner and outer membranes, thereby conferring resistance. These transporters consist of inner (IMP) and outer (OMP) membrane proteins, which are coupled by a periplasmic membrane fusion (MFP) protein. However, it is not know whether the MFP translocates the drug between the membranes, by acting as a channel, or whether it brings the IMP and OMP together, facilitating drug transfer. The MFP EmrA has an elongated periplasmic domain, which binds transported drugs, and is anchored to the inner membrane by a single alpha-helix, which contains a leucine zipper dimerization domain. Consistent with CD and hydrodynamic analyses, the periplasmic domain is predicted to be composed of a beta-sheet subdomain and an alpha-helical coiled-coil. We propose that EmrA forms a trimer in which the coiled-coils radiate across the periplasm, where they could sequester the OMP TolC. The "free" leucine zipper in the EmrA trimer might stabilize the interaction with the IMP EmrB, which also possesses leucine zipper motifs in the putative N- and C-terminal helices. The beta-sheet subdomain of EmrA would sit at the membrane surface adjacent to the EmrB, from which it receives the transported drug, inducing a conformational change that triggers the interaction with the OMP.     

Identification of the auto-inhibitory domains of Aurora-A kinase

Aurora-A is a centrosome-localized serine/threonine kinase that is overexpressed in multiple human cancers. Here, we report an intramolecular inhibitory regulation in Aurora-A between its N-terminal regulatory domain (aa 1-128, Nt) and the C-terminal catalytic domain (aa 129-403, Cd). Removal of Nt results in a significant increase in kinase activity. Nt inhibited the activity of the single C-terminal kinase domain, but had little effect on the activity of the full-length of Aurora-A. PP1 is not involved in this regulation, instead, Nt interacts Cd directly in vitro and in vivo. The non-Aurora box (aa 64-128) in the N-terminal negatively regulated the kinase activity of the C-terminal kinase domain by intramolecular interaction with aa 240-300 within the C-terminal.     

Regulation of c-Fes tyrosine kinase and biological activities by N-terminal coiled-coil oligomerization domains

The cytoplasmic protein-tyrosine kinase Fes has been implicated in cytokine signal transduction, hematopoiesis, and embryonic development. Previous work from our laboratory has shown that active Fes exists as a large oligomeric complex in vitro. However, when Fes is expressed in mammalian cells, its kinase activity is tightly repressed. The Fes unique N-terminal sequence has two regions with strong homology to coiled-coil-forming domains often found in oligomeric proteins. Here we show that disruption or deletion of the first coiled-coil domain upregulates Fes tyrosine kinase and transforming activities in Rat-2 fibroblasts and enhances Fes differentiation-inducing activity in myeloid leukemia cells. Conversely, expression of a Fes truncation mutant consisting only of the unique N-terminal domain interfered with Rat-2 fibroblast transformation by an activated Fes mutant, suggesting that oligomerization is essential for Fes activation in vivo. Coexpression with the Fes N-terminal region did not affect the transforming activity of v-Src in Rat-2 cells, arguing against a nonspecific suppressive effect. Taken together, these findings suggest a model in which Fes activation may involve coiled-coil-mediated interconversion of monomeric and oligomeric forms of the kinase. Mutation of the first coiled-coil domain may activate Fes by disturbing intramolecular coiled-coil interaction, allowing for oligomerization via the second coiled-coil domain. Deletion of the second coiled-coil domain blocks fibroblast transformation by an activated form of c-Fes, consistent with this model. These results provide the first evidence for regulation of a nonreceptor protein-tyrosine kinase by coiled-coil domains.     

Identification of domains of the human papillomavirus type 11 E1 helicase involved in oligomerization and binding to the viral origin

The E1 helicase of papillomavirus is required, in addition to host cell DNA replication factors, during the initiation and elongation phases of viral episome replication. During initiation, the viral E2 protein promotes the assembly of enzymatically active multimeric E1 complexes at the viral origin of DNA replication. In this study we used the two-hybrid system and chemical cross-linking to demonstrate that human papillomavirus type 11 (HPV11) E1 can self-associate in yeast and form hexamers in vitro in a reaction stimulated by single-stranded DNA. Self-association in yeast was most readily detected using constructs spanning the E1 C-terminal domain (amino acids 353 to 649) and was dependent on a minimal E1-E1 interaction region located between amino acids 353 and 431. The E1 C-terminal domain was also able to oligomerize in vitro but, in contrast to wild-type E1, did so efficiently in the absence of single-stranded DNA. Sequences located between amino acids 191 and 353 were necessary for single-stranded DNA to modulate oligomerization of E1 and were also required, together with the rest of the C terminus, for binding of E1 to the origin. Two regions within the C-terminal domain were identified as important for oligomerization: the ATP-binding domain and region A, which is located within the minimal E1-E1 interaction domain and is one of four regions of E1 that is highly conserved with the large T antigens of simian virus 40 and polyomavirus. Amino acid substitutions of highly conserved residues within the ATP-binding domain and region A were identified that reduced the ability of E1 to oligomerize and bind to the origin in vitro and to support transient DNA replication in vivo. These results support the notion that oligomerization of E1 occurs primarily through the C-terminal domain of the protein and is allosterically regulated by DNA and ATP. The bipartite organization of the E1 C-terminal domain is reminiscent of that found in other hexameric proteins and suggests that these proteins may oligomerize by a similar mechanism.     

Identification of PECAM-1 association with sphingosine kinase 1 and its regulation by agonist-induced phosphorylation

Sphingosine 1-phosphate (S1P) is a bioactive lipid mediator generated from sphingosine by sphingosine kinase (SPHK). S1P acts both extracellularly and intracellularly as a signaling molecule, although its intracellular targets are still undefined. Intracellular level of S1P is under strict regulatory control of SPHK regulation, S1P degradation, and S1P dephosphorylation. Therefore, clarifying the mechanisms regulating SPHK activity may help us understand when and where S1P is generated. In this study, we performed yeast two-hybrid screening to search for SPHK1a-binding molecules that may be involved in the regulation of the kinase localization or activity. Platelet endothelial cell adhesion molecule-1 (PECAM-1) was identified as a protein potentially associating with SPHK1a. Their association was confirmed by co-immunoprecipitation analysis using HEK293 cells overexpressing PECAM-1 and SPHK1a. Moreover, the kinase activity appeared to be reduced in stable PECAM-1-expressing cells. PECAM-1 is expressed on the cell surface of vascular cells, and several stimuli are known to induce phosphorylation of its tyrosine residues. We found that such phosphorylation attenuated its association with SPHK1a. This association/dissociation of SPHK with PECAM-1, regulated by the phosphorylated state of the membrane protein, may be involved in the control of localized kinase activity in certain cell types.     

Centaurin beta1 down-regulates nucleotide-binding oligomerization domains 1- and 2-dependent NF-kappaB activation

Centaurin beta1 (CENTB1), a GTPase-activating protein, is a member of the ADP-ribosylation factor family encoded by a gene located on the short arm of human chromosome 17. A yeast two-hybrid screen first suggested a direct interaction between CENTB1 and NOD2. Co-immunoprecipitation experiments confirmed direct interaction between CENTB1 and NOD2 and demonstrated similar interaction between CENTB1 and NOD1. We also demonstrate that endogenous CENTB1 interacts with endogenous NOD2 and NOD1 in SW480 and HT-29 intestinal epithelial cells. CENTB1 partially co-localized with NOD2 and NOD1 proteins in the cytoplasm of mammalian cells. CENTB1 expression in epithelial cells was highly induced by tumor necrosis factor alpha, interleukin 1beta, and the NOD1 and NOD2 ligands (gamma-d-glutamyl-meso-diaminopimelic acid and muramyl dipeptide, respectively). In addition, CENTB1 mRNA level is increased in the inflamed mucosa of patients with inflammatory bowel disease. Functionally, CENTB1 overexpression inhibited NOD1- and NOD2-dependent activation of NF-kappaB, whereas small inhibitory RNA against CENTB1 increased NF-kappaB activation following NOD1- or NOD2-mediated recognition of the bacterial components gamma-d-glutamyl-meso-diaminopimelic acid and muramyl dipeptide, respectively. In contrast, CENTB1 had no effect on NF-kappaB activation induced by Toll-like receptors. In conclusion, CENTB1 selectively down-regulates NF-kappaB activation via NODs pathways, creating a "feedback" loop and suggesting a novel role of CENTB1 in innate immune responses to bacteria and inflammatory responses.     

Myotonic dystrophy protein kinase domains mediate localization, oligomerization, novel catalytic activity, and autoinhibition

Human myotonic dystrophy protein kinase (DMPK) is a member of a novel class of multidomain protein kinases that regulate cell size and shape in a variety of organisms. However, little is currently known about the general properties of DMPK including domain function, substrate specificity, and potential mechanisms of regulation. Two forms of the kinase are expressed in muscle, DMPK-1 and DMPK-2. We demonstrate that the larger DMPK-1 form (the primary translation product) is proteolytically cleaved near the carboxy terminus to generate the smaller DMPK-2 form. We further demonstrate that the coiled-coil domain is required for DMPK oligomerization; coiled-coil mediated oligomerization also correlated with enhanced catalytic activity. DMPK was found to exhibit a novel catalytic activity similar to, but distinct from, related protein kinases such as protein kinase C and A, and the Rho kinases. We observed that recombinant DMPK-1 exhibits low activity, whereas the activity of carboxy-terminally truncated DMPK is increased approximately 3-fold. The inhibitory activity of the full-length kinase was mapped to what appears to be a pseudosubstrate autoinhibitory domain at the extreme carboxy terminus of DMPK. To date, endogenous activators of DMPK are unknown; however, we observed that DMPK purified from cells exposed to the G protein activator GTP-gamma-S exhibited an approximately 2-fold increase in activity. These results suggest a general model of DMPK regulation with two main regulatory branches: short-term activation of the kinase in response to G protein second messengers and long-term activation as a result of proteolysis.     

Identification of the coiled-coil domains of Enterococcus faecalis DivIVA that mediate oligomerization and their importance for biological function

Bacillus subtilis (Bs) DivIVA comprises coiled-coil structures and self-associates forming a 10-12 mer complex in vitro. Using bioinformatic approaches, we determined that Enterococcus faecalis (Ef) DivIVA comprises four coiled-coil domains, one at the N-terminus, the second and the third in the central region of the protein and the fourth at the C-terminus. We determined that DivIVA(Ef) self-interacts and forms a 10-12 multimeric complex. Point mutations or deletions of the central regions predicted bioinformatically to disrupt the coiled-coil structures either eliminated or weakened DivIVA(Ef) self-interaction and reduced oligomerization. Mutations disrupting the N- and C-terminal coiled-coils of DivIVA(Ef) did not affect DivIVA(Ef) oligomerization. The introduction of DivIVA(Ef) mutations to both the N-terminal and the central coiled-coil domains were lethal unless rescued by expressing wild-type DivIVA(Ef) in trans. E. faecalis cells expressing these mutations displayed aberrant cell morphology, indicating disruption of the normal cell division phenotype. The results in E. faecalis also indicate that both the N-terminal and the central coiled-coil structures of DivIVA(Ef) are indispensable for proper biological function. Overexpression of wild-type DivIVA(Ef) in both rod-shaped and round Escherichia coli cells resulted in morphological changes, while the overexpression of DivIVA(Ef) mutations failed to induce such alterations.     

Identification and primary structure of calmodulin binding domains in the phosphorylase kinase holoenzyme

Fast twitch skeletal muscle phosphorylase kinase was isolated and incubated with a radioactive, bifunctional, photoactivable, and cleavable cross-linker conjugated to calmodulin. Incubation of the holoenzyme only resulted in the labeling of the alpha-subunit in the presence of Ca2+. After cleavage with CNBr (and subdigestion with Asp-N protease), a sequence was identified (residues 1069-1087) in the alpha-subunit which had the predominant basic character and the propensity to form an amphiphilic helix like other calmodulin binding domains. If cross-linked calmodulin was incubated with the isolated subunits of phosphorylase kinase, radioactivity was recovered in seven CNBr peptides: three came from the alpha-subunits, one of them corresponding to the sequence labeled in the holoenzyme. Three came from the beta-subunit, and one came from the gamma-subunit. The latter contained the two adjacent calmodulin binding domains recently identified in the gamma-subunit (Dasgupta, M., Honeycutt, T., and Blumenthal, D. K. (1988) J. Biol. Chem. 264, 17156-17163).     

A specific intermolecular association between the regulatory domains of a Tec family kinase

Interleukin-2 tyrosine kinase (Itk), is a T-cell specific tyrosine kinase of the Tec family. We have examined a novel intermolecular interaction between the SH3 and SH2 domains of Itk. In addition to the interaction between the isolated domains, we have found that the dual SH3/SH2 domain-containing fragment of Itk self-associates in a specific manner in solution. Tec family members contain the SH3, SH2 and catalytic domains common to many kinase families but are distinguished by a unique amino-terminal sequence, which contains a proline-rich stretch. Previous work has identified an intramolecular regulatory association between the proline-rich region and the adjacent SH3 domain of Itk. The intermolecular interaction between the SH3 and SH2 domains of Itk that we describe provides a possible mechanism for displacement of this intramolecular regulatory sequence, a step that may be required for full Tec kinase activation. Additionally, localization of the interacting surfaces on both the SH3 and SH2 domains by chemical shift mapping has provided information about the molecular details of this recognition event. The interaction involves the conserved aromatic binding pocket of the SH3 domain and a newly defined binding surface on the SH2 domain. The interacting residues on the SH2 domain do not conform to the consensus motif for an SH3 proline-rich ligand. Interestingly, we note a striking correlation between the SH2 residues that mediate this interaction and those residues that, when mutated in the Tec family member Btk, cause the hereditary immune disorder, X-linked agamaglobulinemia.